CI: add the grabpl build-backend command into the repo (#52673)

* Move the grabpl build-backend command and clean it up a bit

pkg/build/cmd/flags.go

@@ -0,0 +1,23 @@
+package main
+
+import "github.com/urfave/cli/v2"
+
+var (
+	jobsFlag = cli.IntFlag{
+		Name:  "jobs",
+		Usage: "Number of parallel jobs",
+	}
+	buildIDFlag = cli.StringFlag{
+		Name:  "build-id",
+		Usage: "Optionally supply a build ID to be part of the version",
+	}
+	editionFlag = cli.StringFlag{
+		Name:  "edition",
+		Usage: "The edition of Grafana to build (oss or enterprise)",
+		Value: "oss",
+	}
+	variantsFlag = cli.StringFlag{
+		Name:  "variants",
+		Usage: "Comma-separated list of variants to build",
+	}
+)