Nix workflow broken on macos-latest (macos-15-arm64)

[andreasabel]

https://github.com/haskell/hackage-server/actions/runs/17315560040/job/49157667263#step:3:97

mv: fastcopy: open() failed (to): /usr/bin/rmz: Read-only file system

Upstream issue:

• Fails on macos-15-arm64: /usr/bin/rmz: Read-only file system wimpysworld/nothing-but-nix#39

CC @peterbecich

[peterbecich]

The immediate issue caused by wimpysworld/nothing-but-nix#39 is solved. However, there are other problems with the Nix Flake on Mac.