ci/cd: prevent multiple workflows for the same code

[m1rm]

This change prevents multiple runs for the same code, which can happen if someone pushes changes quick enough after a previous push.

See also: https://docs.github.com/en/actions/how-tos/write-workflows/choose-when-workflows-run/control-workflow-concurrency

[jelly]

This feels like a very niche situation, this repository has very low activity so it doesn't feel worth it to optimise for it.

[m1rm]

This feels like a very niche situation, this repository has very low activity so it doesn't feel worth it to optimise for it.

I understand :) just thought it's a nice small thing but yeah, it is propably not necessary