ci: per repo concurrency group

[romanc]

Description

Quick follow-up from #91: 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.

Related: NOAA-GFDL/pySHiELD#70.

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 changed was triggered by a model need/shortcoming: N/A