ci: specialize concurrency group per repository

[romanc]

Description

Quick follow-up from #69: specialize concurrency group per repository. Otherwise, we'll have deadlocks for concurrency groups and the cache won't be built:

Link to the above workflow.

Note: using ${{ github.repository }} sounds like a good idea. In practice, that doesn't play nice when the workflow is called from another repository because in that case, github.repository resolves to the calling repository.

How Has This Been Tested?

By staring at the code. I'll verify post-merge that this works as expected.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A
• Targeted model, if this change was triggered by a model need/shortcoming: N/A