version: Add versioning for github tarballs

When building from github tarballs, getting the version from `git
describe` won't work, or worse, will be irrelevant [1]

However, we can make use of $Format$ and .gitattributes as described in
[2] to automatically substitute the correct version in a go variable
when `git archive` is used.

In particular, the correct version number will automatically be
substituted when GitHub creates release tarballs.

[1] rpms/gvisor-tap-vsock.spec uses `%autosetup -Sgit -n %{name}-%{version}` which unpacks
the release tarball in a newly created git repository.

[2] https://icinga.com/blog/2022/05/25/embedding-git-commit-information-in-go-binaries/

Signed-off-by: Christophe Fergeau <[email protected]>

pkg/cmdline/version.go

@@ -2,13 +2,23 @@ package cmdline
 
 import (
 	"runtime/debug"
+	"strings"
 )
 
-// set using the '-X github.com/crc-org/vfkit/pkg/cmdline.gitVersion' linker flag
-var gitVersion = "unknown"
+var (
+	// set using the '-X github.com/crc-org/vfkit/pkg/cmdline.gitVersion' linker flag
+	gitVersion = "unknown"
+
+	// set through .gitattributes when `git archive` is used
+	// see https://icinga.com/blog/2022/05/25/embedding-git-commit-information-in-go-binaries/
+	gitArchiveVersion = "$Format:%(describe)$"
+)
 
 func Version() string {
 	switch {
+	// This will be substituted when building from a GitHub tarball
+	case !strings.HasPrefix(gitArchiveVersion, "$Format:"):
+		return gitArchiveVersion
 	// This will be set when building from git using make
 	case gitVersion != "":
 		return gitVersion