Fix GHA CI for PRs from external contributors

[jepio]

Fix GHA CI for PRs from external contributors

GHA CI fails for PRs from external contributors because in devcontainer tests, emerge-gitclone can't fetch the right ref from the upstream repository. We can make this work by:

1. switching to encoding a full sha in the FLATCAR_VERSION string in GHA
2. using that full sha with git fetch origin <sha>; git checkout FETCH_HEAD in emerge-gitclone

This PRs does "1." and switches to using a branch of flatcar-dev-util that does "2." (https://github.com/flatcar/flatcar-dev-util/compare/jepio/fetch-head - PR will follow if this works).

How to use

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Testing done

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• [ ] Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• [ ] Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.