mac arm builds

[aryairani]

Overview

Add some CI support for building on multiple platforms besides x86_64.

Adds a new runner for releases, and changes the filenames of release packages to include an architecture tag:

• ucm-linux-x64.tar.gz
• ucm-macos-arm64.tar.gz
• ucm-macos-x64.tar.gz
• ucm-windows-x64.zip

Closes #3045

Interesting/controversial decisions

Wasn't sure whether mac arm and x64 both need to be run in CI or just during releases; as of now, it's just x64 in CI, like before.

Test coverage

I ran it here: https://github.com/unisonweb/unison/releases/tag/mac-arm-build-build

Loose ends

[aryairani]

When this gets merged, scripts that try to download the latest release or the nightly release will need to update the file name they're looking for, to include the architecture per the bullets in the description cc @ceedubs @rlmark

[kubukoz]

When this gets merged, scripts that try to download the latest release or the nightly release will need to update the file name they're looking for, to include the architecture per the bullets in the description cc @ceedubs @rlmark

This will also affect Nixpkgs: https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/compilers/unison/default.nix