GitHub Actions: add back an x86-64_darwin runner

[roberth]

Motivation

x86_64-darwin build had an issue.

Context

• Catches libmain: add missing header include #11138

Priorities and Process

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[emilazy]

Thanks! Note that you could potentially get faster CI by still using an AArch64 runner and just passing --system x86_64-linux, which works seamlessly with Rosetta 2. It depends on what hardware GitHub uses for runners, though.

[emilazy]

(Probably that third commit isn’t meant to be here?)

[roberth]

(Probably that third commit isn’t meant to be here?)

That was for troubleshooting this failure

EDIT: which happened again on e48e0cb

[roberth]

Interestingly hydra doesn't experience these build failures, so I think we just don't support macOS 13 anymore.
Unfortunately that's the only version for which GitHub has free runners that are native x86_64 machines.
This is not a requirement for the build though, so perhaps we could build with Rosetta, and then perform the installer test on a native machine. Either way, we'll have to be more explicit about the platforms we support in doc/manual/src/installation/supported-platforms.md

[emilazy]

Dropping support for macOS 13 would be pretty brutal since it’s been at 10.12 for the longest time and Apple supports back to macOS 12. We definitely don’t currently have consensus on the Nixpkgs side to even drop support for pre‐macOS 11 (although I’d personally support such a move). I’m not sure why the runner OS version would matter that much, though; the build goes through the Nixpkgs machinery, right? In which case it should still be using an old SDK no matter what OS version it’s being built on.