ci: Transition to self-hosted runners

[samuelburnham]

• Removes arm64 job on CircleCI as we are already testing on ARM via M1 Mac, and I'm not sure we need first-class support for ARM on Linux.
• Moves linux_release CircleCI job to self-hosted GH Actions as linux_ignored
• Runs above linux_ignored on merge_group only, as it is expensive to run and seems wasteful to trigger on every PR commit. This is a speculative change and fine to revert, but it may cause bottlenecks on the self-hosted runners.
• Moves the nightly linux_exhaustive CircleCI job to self-hosted GH Actions. The cron job currently runs at midnight UTC time, perhaps it would be better in a timezone like ET?
• Splits CUDA tests into parallel CUDA and OpenCL builds to use scaled GPU runners, and removes the dependency on CC=gcc-10 CXX=g++-10 due to the new CUDA-enabled Docker image in Switch to CUDA base image gh-actions-runner#12. This closes ci: GPU runners require CC=gcc-10 CXX=g++-10 with CUDA 11.5 #597

Fixes #504

[huitseeker]

I'm OK with all those tests, except perhaps the removal of ARM linux:

• it may be useful for cloud runners
• it already allowed us to find issues with dependencies
  Pinging @porcuquine for an opinion

[porcuquine]

All else equal, I don't favor reducing test surface — especially if this has actually uncovered errors. There was also this: supranational/blst@4f9d557#diff-78a8613e47fd3a587897556bdbcaf5c3b1bd53400cbd57f01322c98f2ebe1147 — thought that was indeed M1.

If we're certain the M1 coverage would catch all the same bugs of this category, and the cost savings warrant it, then removing seems okay. If the cost is relatively minor, I'd generally favor keeping it. I think given the full range of use cases, Lurk on a raspberry pi is not even entirely far-fetched — even if not used for heavy proving…

[porcuquine]

And yes, to @huitseeker's point about cloud hardware. Basically, I think explicit support is worthwhile.

[huitseeker]

Thanks a lot!

CI failures are a bit concerning

[huitseeker]

Once more unto the breach ...