Suggested improvements for R&D CI runs

[climbfuji]

Is your feature request related to a problem? Please describe.

See #940 (review):

Would it make sense to use strategy:matrix:... to avoid all the indentation, and also allow gcc and intel to run in parallel? I have no problem with the current approach though.

Describe the solution you'd like
See above

Additional context
n/a

[climbfuji]

I thought about this a bit more. Because that's running on a self-hosted runner in the same directory, we'd have to make a lot more efforts to make sure we can build two separate environments (append compiler to env name?) using the same temporary directories etc. I want to avoid this. My hope is that we can revert all this when the problem goes away in the future (either by itself, same as it showed up first, or because the spack developers implement the hierarchical matrix approach I mentioned earlier). So I am going to fix the bug that I introduced in the R&D CI in a PR shortly, but leave the logic as is if that's ok.