Adds actionlint to lint GitHub Actions workflows

[JayShortway]

Since we have a relatively complex GitHub Actions workflow, I figured it makes sense to add a linter to automatically catch errors. I've been using the online variant of this linter any time some workflow isn't working: https://rhysd.github.io/actionlint/. The same was recently merged to purchases-kmp: RevenueCat/purchases-kmp#227.

[JayShortway]

Back to draft, because repository rules restrict usage of actionlint.

[JayShortway]

This is ready for review. If it's approved, I'll update the repository settings to allow actionlint (and GHA's checkout action). If you disagree, we can close this. No biggie.

[tonidero]

LGTM!

[JayShortway]

@tonidero @aboedo Had to make these changes to trigger_all_tests.yml to make the linter happy:

-        run: echo ::set-output name=branch::$(gh pr view $PR_NO --repo $REPO --json headRefName --jq '.headRefName')
+        run: echo "branch=$(gh pr view "$PR_NO" --repo "$REPO" --json headRefName --jq '.headRefName')" >> "$GITHUB_OUTPUT"

1. Not using the deprecated ::set-output (docs).
2. Double quoting variables ($PR_NO & $REPO) to prevent globbing and word splitting.

Please have a looksie.