CI: populate nix cache for aarch64-linux

[ceedubs]

Currently the nix flake marks aarch64-linux as a supported system, but our Nix development cache CI job doesn't publish to the nix cache for aarch64-linux. When I attempted to make it publish for ARM linux, the build repeatedly timed out after an hour; no incremental results seem to be saved.

See this discussion.

[sellout]

Yeah, it would be nice if we could cache incrementally, so then repeated builds would make progress.

For the time being, I’ve relied on pushing to the cache manually after building locally (#5795). The issue is having someone who has both some interest in the Nix build and a machine that’ll build aarch64-linux.

[aryairani]

Github now has aarch64-linux runners

[sellout]

The problem is that the initial build will build GHC, etc. and time out on GiHub, so someone has to do an aarch64-linux build elsewhere, then push to Cachix so GitHub can just fetch everything.

[ceedubs]

One thing that I've noticed but haven't been able to understand is that the builds time out after an hour but the GitHub actions docs say that the timeout for a job is higher (I think 6 hours?). And in my unison-nix repo it gets through a 2+ hour build of ucm on aarch64-linux runners https://github.com/ceedubs/unison-nix/actions/runs/18012578621