ci: improve CI caching scheme

[lgritz]

I recently discovered that the caching of our ccache was either not working at all, or was much less efficient/heplful than I thought. So there are a number of improvements here:

• Split caching into separate restore/save, and rearrange the step order so we save cache before tests run. This allows us to save the ccache even for jobs with failed tests (where the all-in-one unsplit approach would skip the cache save step for a failed job), or for jobs that we purposely end early (maybe because we see failing tests), not only when all tests finish and succeed. This makes subsequent builds, that we do on the same branch while attempting to fix failures, go faster by using the cache.

• Make subsequent pushes to the same PR or REF to cancel any previous jobs, so if we push updates before the last CI run of the same branch completes, it won't keep those going as useless zombie jobs.

• Change the cache key names we use to remove the github ref to encourage cache reuse across different commits or branches (I think), and to fix a couple errors where we had mis-named things and were sharing caches between jobs when we shouldn't or vice versa.

• Be more careful about the location of the cache, making sure we were for sure telling the GHA cache action to be saving/restoring the same directory location that we told ccache to use as its cache location.

• Use CCACHE_COMPRESSION env var to be extra sure we're using compressed caches to be sure we get the best bang for buck on the limited cache storage we're allowed.

In all, there were a number of subtle things broken. Give it a good scrubbing. I'm not really sure exactly which one of these, or which combination, was most responsible for getting things unstuck, but it definitely works! Some results from two test pushes in a row I did after these changes:

test                        build deps     build osl
-----------------------     ------------   ----------------------
VFXP 2024 / cold            4:12            6:11
VFXP 2024 / cached          1:42 (2.5x)     0:48 (7x)

ABI check / cold            4:11            6:14 + 5:38(abi ref)
ABI check / cached          0:57 (4.4x)     0:46 + 0:16 (11.5x)

bleeding edge / cold        8:12            9:57
bleeding edge / cached      1:39 (5x)       0:44 (13.6x)

It varies across jobs and from run to run, but let's call it something in the neighbourhood of 4x speedup building dependencies and 10x building OSL itself. It's not better because it doesn't speed up linking, and not all of the dependencies' build systems are doing things in a way that is amenable to ccache inserting itself into the process. Still, this should be a big help in reducing the time of our CI runs.

[lgritz]

Seem ok? I would love to get this merged because once it's in, all other CI runs should be significantly faster.