CI: Only run with nix remote for ubuntu

Previously, there would be jobs for macos-12 having `withNixRemote = true`, but without having any effect.

.github/workflows/workflow.yaml

@@ -29,6 +29,9 @@ jobs:
         withNixRemote:
           - true
           - false
+        exclude:
+          - os: macos-12
+            withNixRemote: true
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
@@ -39,7 +42,7 @@ jobs:
         with:
           buildbuddy_api_key: ${{ secrets.BUILDBUDDY_API_KEY }}
       - name: Start remote Nix server
-        if: matrix.withNixRemote && matrix.os == 'ubuntu-latest'
+        if: matrix.withNixRemote
         run: |
           # Generate temporary SSH keys.
           mkdir -p $HOME/.ssh
@@ -52,16 +55,13 @@ jobs:
           cp .github/nix-server/keys/* $HOME/.ssh/
 
           sudo cp .github/nix-server/config /etc/ssh/ssh_config
+
+          echo "Setting BAZEL_NIX_REMOTE env variable"
+          echo BAZEL_NIX_REMOTE=nix-server >> "$GITHUB_ENV"
       - name: Build & test
         env:
           BZLMOD_ENABLED: ${{ matrix.bzlmodEnabled }}
-          NIX_REMOTE_ENABLED: matrix.withNixRemote && matrix.os == 'ubuntu-latest'
         run: |
-          if [ "$NIX_REMOTE_ENABLED" = "true" ]; then
-            echo "Setting BAZEL_NIX_REMOTE env variable"
-            export BAZEL_NIX_REMOTE=nix-server
-          fi
-
           nix-shell --pure \
             --keep GITHUB_STEP_SUMMARY \
             --keep GITHUB_REPOSITORY \