Test using macos-15

[keiravillekode]

No description provided.

[BNAndras]

It looks like we're trying to download an macOS arm64 release, but one isn't available for the pinned nightly.

I poked around a little, and it seems that platform wasn't targeted until earlier this year. https://github.com/nim-lang/nightlies/releases/tag/2025-02-01-version-2-0-1ff69eae17388be06221369b2bb08f8808905472 is an option if we're fine targeting 2.0.15.

[ee7]

Thanks for the PR. I'm not against this, now that Nim recently starting providing arm64 releases for macOS, and after we first merge:

• .github: bump iffy/install-nim from 5.0.10 to 5.1.0 #625
• .github: bump Nim from 2.0.0 to 2.2.4 #622

However, I'd like to note that this PR is not just a bump of OS version and dependencies; it changes the architectures that we're testing. I might lean slightly towards simply adding arm64 macOS to the testing matrix, rather than replacing x86_64 macOS. I was also going to get around to adding arm64 Linux and Windows.

The current runner labels and their associated runners are documented here. The macOS labels are not particularly intuitive:

• x86_64: macos-13 (AKA macos-13-large), macos-14-large, and macos-15-large
• arm64: macos-14 (AKA macos-14-xlarge), and macos-15 (AKA macos-15-xlarge)

I suppose it was done like that to optimize for cost at the expense of making the upgrades macos-13 -> macos14 or macos-13 -> macos-15 a non-obvious architecture change.

Furthermore, I was not a fan of the fact that people who used macos-latest got switched from x86_64 to arm64 underneath their feet. I'm glad I pin to specific labels everywhere.