Add a GitHub Actions workflow for ARM64

[snoyberg]

This also updates release.hs to use a more recent resolver, since
otherwise we'll have trouble on ARM64 with conflicting LLVM
installations. Unlike #5552, I took the simple workaround here of
installing GHC 8.6.5 in addition to the more recent GHC needed by
release.hs.

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[snoyberg]

The approach I took with modifying the installed GHC didn't work, so I'm going to take another approach instead. I've separated off the sanity check PR to #5556. #5552 can continue to deal with bumping the resolver.

Also, I'm not happy with how much this approach relies on the setup of the GitHub runner instance, so I'm going to rework the ARM installation to use Docker instead.

[snoyberg]

@borsboom @qrilka this is ready for review now. I've tested, and the ARM64 bindist seems to work correctly.