fix workflow concurrency

[proppy]

When multiple instance of the user-project-ci workflow runs concurrently:

• the first one to complete will push the new hardnened gds to the main branch.
• this causes the latest one to finish to fail the commit job (git push fails because the main remote now has new commits that didn't exist at the time the run started)

see https://github.com/proppy/tiny_user_project/actions/runs/3545298578

One way to fix this could be to rebase before pushing, or use https://docs.github.com/en/actions/using-jobs/using-concurrency to prevent multiple runs of the same workflow to happen concurrently.

[mattvenn]

https://github.com/mattvenn/gf180-tiny-7seg/actions/runs/3602495979