Use Nix platform identifiers for release binaries

[amesgen]

Recently, macos-latest switched from macos-12 (x86_64) to macos-14 (aarch64, i.e. M1 ARM). The current code for building a macOS release binary only works for x86_64-darwin. This PR fixes that (allowing to also build on aarch64-darwin), and also renames the release artifacts from

• ormolu-Linux.zip
• ormolu-macOS.zip
• ormolu-Windows.zip

to the more precise

• ormolu-x86_64-linux.zip
• ormolu-aarch64-darwin.zip
• ormolu-x86_64-darwin.zip
• ormolu-x86_64-windows.zip

See https://github.com/amesgen/ormolu/releases/tag/test-release-new for an example release with this new code.

Also, we now properly sign the macOS binaries, as running aarch64-darwin binaries requires that.

[github-actions[bot]]

🚀 Deployed on https://140c0a6754ae1d87f4fc79e4ddbdfd63edf18f1c--ormolu-live.netlify.app

[mrkkrp]

Looks great, thanks!