Add nix CI (#28036)

This adds a nix CI job to build the flake in debug mode for
aarch64-darwin and x86-linux. For now this job will only run when the
`run-nix` label is added to a PR.

The CI job doesn't push to cachix for now, so every build is a clean
build.

I also added a condition to the garbage collection step so it only runs
when the nix store is >50GB.

Release Notes:

- N/A

.github/workflows/release_nightly.yml

@@ -216,7 +216,8 @@ jobs:
           name: zed-industries
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
       - run: nix build
-      - run: nix-collect-garbage -d
+      - name: Limit /nix/store to 50GB
+        run: '[ $(du -sm /nix/store | cut -f1) -gt 50000 ] && nix-collect-garbage -d'
 
   update-nightly-tag:
     name: Update nightly tag