renovate: Pin GitHub Action digests (#31)

Author: bkeryan

.github/renovate.json

@@ -5,6 +5,7 @@
     "config:recommended",
     ":maintainLockFilesWeekly",
     ":automergePatch",
-    "schedule:automergeDaily"
+    "schedule:automergeDaily",
+    "helpers:pinGitHubActionDigestsToSemver"
   ]
-}
+}