MacOS rebuilds dev shells because no nix cache hits

[locallycompact]

We dropped hydraJobs which was built by some aarch64-darwin runner in the past (IOG CI).

Put the devshell build as a checks entry in the flake again?

[locallycompact]

We no longer use IOG's CI, so we need to push to cachix.

I think the more scalable approach is to have cachix watch on an x86 runner and the darwin as a remote builder. This means only ssh-copy-id needs to be set up via the x86 and darwin machines, rather than installing github-runner and cachix auth on every darwin machine.