arm64 runner improvements

[jeremyd2019]

• use runner groups for self-hosted runners #93
• Switch to new ARM64 Git for Windows
  • It's still beta, but would probably work ok, and we can get it some exercise.
• Switch to ARM64 Python
  • just remove the architecture: x64. I tested, but there are not yet binary wheels for win-arm64, so installing requirements fails for lack of visual studio. I'm not interested in installing visual studio just for that.
• Update to Windows 11 24H2
  • I did some preliminary work (and random commentary) on that in Microsoft improvements for ARM64 in 24H2 jeremyd2019/winautoconfig#5, but I haven't been offered the update yet on any system (though I didn't opt in for latest updates). I'd like to wait until the host gets the update at least. Based on my previous feelings on 'major' Windows upgrades, I'd normally wait a few months for the bugs to be shaken out of the new version.

[lazka]

* [ ]  Switch to ARM64 Python

None of the packages affected provide arm64 binary wheels. They are likely all waiting for GitHub hosted runners (for example pyca/cryptography#11199) as Python packaging does not support cross compiling (except hackily with setuptools which is on the way out). So I don't expect much progress there. The timeline for those runners was recently moved/adjusted to Q1 2025 (github/roadmap#970)