fix(gh-cache): Get remote changes before pushing, to avoid concurrent write failures (#339)

If two concurrent jobs attempt to push to the `gh-cache` branch at the
same time, `git` fails with an error like this:

```
[gh-cache] Save artifact: cached_findings-main.json
 1 file changed, 1 insertion(+), 1 deletion(-)
Committed 'cached_findings-main.json' to 'gh-cache' branch
To https://github.com/github-community-projects/continuous-ai-for-accessibility-scanner
 ! [rejected]            gh-cache -> gh-cache (fetch first)
error: failed to push some refs to 'https://github.com/github-community-projects/continuous-ai-for-accessibility-scanner'
hint: Updates were rejected because the remote contains work that you do not
hint: have locally. This is usually caused by another repository pushing to
hint: the same ref. If you want to integrate the remote changes, use
hint: 'git pull' before pushing again.
```

This happens even in cases where the remote’s changes and the local
changes don’t conflict, e.g. if two different files were updated.

This PR fixes the issue by fetching and rebasing[^1] before pushing.

[^1]: Unless the `gh-cache` branch doesn’t existing on the remote. If
this is the case, the commands are skipped, because a. they are
unnecessary and b. they would fail.

Author: smockle

.github/actions/gh-cache/save/action.yml

@@ -45,7 +45,7 @@ runs:
         token: ${{ inputs.token }}
         fetch-depth: 0
         path: .gh-cache-${{ github.run_id }}
-        show-progress: 'false'
+        show-progress: "false"
 
     - name: Switch to gh-cache branch (or create orphan)
       shell: bash
@@ -79,6 +79,10 @@ runs:
           git commit -m "$(printf 'Save artifact: %s' "${{ inputs.path }}")" \
             && echo "Committed '${{ inputs.path }}' to 'gh-cache' branch" \
             || echo "No changes to commit"
+          if git ls-remote --exit-code --heads origin gh-cache >/dev/null; then
+            git fetch origin gh-cache
+            git rebase origin/gh-cache
+          fi
           git push origin gh-cache
         else
           echo "'${{ inputs.path }}' does not exist"