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

* move grafana-build into pkg/build

pkg/build/daggerbuild/stringutil/random.go

@@ -0,0 +1,17 @@
+package stringutil
+
+import (
+	"math/rand"
+	"time"
+)
+
+var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
+
+func RandomString(n int) string {
+	r := rand.New(rand.NewSource(time.Now().UnixNano()))
+	b := make([]rune, n)
+	for i := range b {
+		b[i] = letters[r.Intn(len(letters))]
+	}
+	return string(b)
+}