feat: Danger - check for unpinned actions

[vaind]

In response to getsentry/sentry-java#1442 - actions should be pinned to reduce attack surface.

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Danger - check for unpinned actions ([#39](https://github.com/getsentry/github-workflows/pull/39))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

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