CI: move grafana-build into pkg/build (#105640)

* move grafana-build into pkg/build

pkg/build/daggerbuild/daggerutil/hostdir.go

@@ -0,0 +1,22 @@
+package daggerutil
+
+import (
+	"errors"
+	"os"
+
+	"dagger.io/dagger"
+)
+
+// HostDir checks that the directory at 'path' exists and returns the dagger.Directory at 'path'.
+func HostDir(d *dagger.Client, path string) (*dagger.Directory, error) {
+	info, err := os.Stat(path)
+	if err != nil {
+		return nil, err
+	}
+
+	if !info.IsDir() {
+		return nil, errors.New("given hostdir is not a directory")
+	}
+
+	return d.Host().Directory(path), nil
+}