ci: pin actions to node20-safe tags + runner sock pass-through

forgejo-runner v6.4.0 ships a node20 javascript engine. v4.2+ of
actions/checkout and v2.0.5+ of softprops/action-gh-release moved to
node24, which the runner refuses to exec. Pin both to last node20
release.

Pairs with a runner-side config change (separately deployed on
nullstone /home/docker/forgejo-runner/conf/config.yaml) that adds
`-v /var/run/docker.sock:/var/run/docker.sock` to per-job container
options + whitelists the socket via valid_volumes — without that
addnab/docker-run-action@v3 inside the catthehacker/ubuntu job
container can't reach the docker engine.

- actions/checkout v4 -> v4.1.7
- softprops/action-gh-release v2 -> v2.0.4
- addnab/docker-run-action v3 unchanged (composite/docker, no node)
- ludeeus/action-shellcheck@master unchanged (docker-based)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/build-iso.yml

@@ -41,6 +41,9 @@ jobs:
           df -h
 
       - name: Run build inside Fedora 43 container
+        # v3 is composite/docker-based — no node runtime in the action
+        # itself. Safe under node20 forgejo-runner. TODO(infra): consider
+        # SHA pinning in a follow-up sweep.
         uses: addnab/docker-run-action@v3
         with:
           image: registry.fedoraproject.org/fedora:43