fix: lock tenant map to avoid race

[cbuto]

Fixes another race saw here:

https://github.com/helm/chartmuseum/actions/runs/5413041445/jobs/9838017827#step:5:6653

WARNING: DATA RACE
Write at 0x00c00099d650 by goroutine 254:
  runtime.mapassign_faststr()
      /opt/hostedtoolcache/go/1.20.5/x64/src/runtime/map_faststr.go:203 +0x0
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServer).initCacheEntry()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/cache.go:345 +0x2fc
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServer).getIndexFile()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/index.go:34 +0xda
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServer).getIndexFileRequestHandler()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/handlers.go:123 +0x1cb
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServer).getIndexFileRequestHandler-fm()
      <autogenerated>:1 +0x44
  helm.sh/chartmuseum/pkg/chartmuseum/router.(*Router).rootHandler()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/router/router.go:250 +0x8c2
  helm.sh/chartmuseum/pkg/chartmuseum/router.(*Router).rootHandler-fm()
      <autogenerated>:1 +0x44
  github.com/gin-gonic/gin.(*Context).Next()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-gonic/gin/context.go:174 +0x207
  github.com/gin-contrib/size.RequestSizeLimiter.func1()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-contrib/size/size.go:88 +0x19b
  github.com/gin-gonic/gin.(*Context).Next()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-gonic/gin/context.go:174 +0x238
  helm.sh/chartmuseum/pkg/chartmuseum/router.requestWrapper.func1()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/router/middleware.go:48 +0x1aa
  github.com/gin-gonic/gin.(*Context).Next()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-gonic/gin/context.go:174 +0x151
  github.com/gin-gonic/gin.CustomRecoveryWithWriter.func1()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-gonic/gin/recovery.go:102 +0xbd
  github.com/gin-gonic/gin.(*Context).Next()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-gonic/gin/context.go:174 +0x109
  github.com/gin-gonic/gin.serveError()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-gonic/gin/gin.go:656 +0x8d
  github.com/gin-gonic/gin.(*Engine).handleHTTPRequest()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-gonic/gin/gin.go:649 +0x8fd
  github.com/gin-gonic/gin.(*Engine).HandleContext()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/gin-gonic/gin/gin.go:587 +0x3b0
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServerTestSuite).doRequest()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/server_test.go:102 +0x62b
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServerTestSuite).testAllRoutes()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/server_test.go:1077 +0x36a
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServerTestSuite).TestRoutes()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/server_test.go:1050 +0x331
  runtime.call16()
      /opt/hostedtoolcache/go/1.20.5/x64/src/runtime/asm_amd64.s:728 +0x48
  reflect.Value.Call()
      /opt/hostedtoolcache/go/1.20.5/x64/src/reflect/value.go:370 +0xc7
  github.com/stretchr/testify/suite.Run.func1()
      /home/runner/work/chartmuseum/chartmuseum/vendor/github.com/stretchr/testify/suite/suite.go:197 +0x70d
  testing.tRunner()
      /opt/hostedtoolcache/go/1.20.5/x64/src/testing/testing.go:1576 +0x216
  testing.(*T).Run.func1()
      /opt/hostedtoolcache/go/1.20.5/x64/src/testing/testing.go:1629 +0x47

Previous read at 0x00c00099d650 by goroutine 335:
  runtime.mapaccess1_faststr()
      /opt/hostedtoolcache/go/1.20.5/x64/src/runtime/map_faststr.go:13 +0x0
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServer).getChartList()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/cache.go:103 +0xf2
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServer).refreshCacheEntry()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/cache.go:595 +0xcb
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServer).rebuildIndexForTenant()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/cache.go:591 +0x397
  helm.sh/chartmuseum/pkg/chartmuseum/server/multitenant.(*MultiTenantServer).rebuildIndex.func2()
      /home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/cache.go:576 +0x58

[kycheng]

@cbuto I also encountered the same problem, will this change be fixed in 0.14.1?