CI: Build pre-release artifacts in GhA on merge to release branches and main. (#106779)

* Add grafana-build action and workflow

* Fix the --verify flag stalling on tar.gz builds

* Add event sources for main / release branches

* Update CODEOWNERS

package.json

@@ -6,7 +6,7 @@
   "version": "12.1.0-pre",
   "repository": "github:grafana/grafana",
   "scripts": {
-    "build": "NODE_ENV=production nx exec --verbose -- webpack --config scripts/webpack/webpack.prod.js --progress",
+    "build": "NODE_ENV=production nx exec --verbose -- webpack --config scripts/webpack/webpack.prod.js",
     "build:nominify": "yarn run build -- --env noMinify=1",
     "build:stats": "NODE_ENV=production webpack --progress --config scripts/webpack/webpack.stats.js",
     "dev": "NODE_ENV=dev nx exec -- webpack --config scripts/webpack/webpack.dev.js",