Try to resolve plugin sign failure

https://github.com/Gowee/traceroute-map-panel/runs/2146736283?check_suite_focus=true

Error:

> Run npx @grafana/toolkit plugin:sign
> Invalid command: plugin:sign
> See --help for a list of available commands.

The problem appears to be that npx attempts to install and then use another
outdated version of @grafana/toolkit in global somehow. The mysterious
behaviours can be reproduced on node:12 altough almost the same workflow
works well when releasing v0.3.0-beta2. Anyway, just wrap
@grafana/toolkit command in package.json commands. Besides, node version
for CI runner is also upgraded, to 15.

.github/workflows/main.yml

@@ -20,7 +20,7 @@ jobs:
     - uses: actions/checkout@v2
     - uses: actions/setup-node@v1
       with:
-        node-version: '12'
+        node-version: '15'
     - run: yarn install
     - run: yarn build
     - run: yarn ci # no effective test so far, just to make sure it lints & builds well

.github/workflows/publish.yml

@@ -24,7 +24,7 @@ jobs:
         git merge "${GITHUB_REF#refs/heads/}" 
     - uses: actions/setup-node@v1
       with:
-        node-version: '12'
+        node-version: '15'
     - name: Init
       run:
         yarn install
@@ -38,7 +38,7 @@ jobs:
       env: # Or as an environment variable
         GRAFANA_API_KEY: ${{ secrets.GRAFANA_API_KEY }}
       run:
-        npx @grafana/toolkit plugin:sign
+        yarn sign
     - name: Create tarball
       run: 
         tar czf /tmp/dist.tar.gz --transform s/^dist/traceroute-map-panel/ dist/

package.json

@@ -8,6 +8,8 @@
     "dev": "grafana-toolkit plugin:dev",
     "watch": "grafana-toolkit plugin:dev --watch",
     "ci": "grafana-toolkit plugin:ci-build",
+    "sign": "grafana-toolkit plugin:sign",
+    "start": "yarn watch",
     "lint": "eslint -c node_modules/@grafana/toolkit/src/config/eslint.plugin.json src --ext js,jsx,ts,tsx"
   },
   "repository": "github:gowee/traceroute-map-panel",