CI Jobs busted on x86_64-darwin, on Monterey and later

I've been trying to figure out why we keep disabling runners and ... it turns out the toolchain cannot be built for x86_64-darwin on Monterey systems.

$ nix build -f .gitlab/darwin/toolchain.nix --argstr system x86_64-darwin -o toolchain.sh
builder for '/nix/store/4gky1zsf3lsbk3bl70krm4mgfirrc16p-ghc.drv' failed with exit code 77; last 10 log lines:
  checking for path to top of build tree... /private/tmp/nix-build-ghc.drv-0/ghc-9.2.2-x86_64-apple-darwin
  checking for a BSD-compatible install... /nix/store/mha2vnicr34nz80mbm9xx561h9wq27nh-coreutils-8.32/bin/install -c
  checking whether ln -s works... yes
  checking for gsed... sed
  checking for python3... no
  checking Xcode version... not found (too old?)
  checking whether the C compiler works... no
  configure: error: in `/private/tmp/nix-build-ghc.drv-0/ghc-9.2.2-x86_64-apple-darwin':
  configure: error: C compiler cannot create executables
  See `config.log' for more details
cannot build derivation '/nix/store/dd0sj8nnnfdbx6kl1y2gzsx3llx128f8-toolchain.drv': 1 dependencies couldn't be built
[5 built (1 failed), 312 copied (2861.2 MiB), 606.6 MiB DL]
error: build of '/nix/store/dd0sj8nnnfdbx6kl1y2gzsx3llx128f8-toolchain.drv' failed

/cc @bgamari @chreekat