ci: group action digest updates with non-major dependencies

Digest and pinDigest updates are not covered by the group:allNonMajor
preset (which only matches minor/patch), so SHA-pinned actions like
goreleaser-action produced standalone PRs. Since these actions are
pinned to floating major tags, a digest update is always within-major
and safe to group. Reuse the preset's groupName/groupSlug so they merge
into the same weekly grouped PR.

.github/renovate.json

@@ -21,6 +21,11 @@
     }
   ],
   "packageRules": [
+    {
+      "matchUpdateTypes": ["digest", "pinDigest"],
+      "groupName": "all non-major dependencies",
+      "groupSlug": "all-minor-patch"
+    },
     {
       "matchManagers": ["github-actions"],
       "addLabels": ["area: github actions"]