ci: add setup-gnu-tools action for macOS runners (#21938)

macOS runners lack GNU toolchain dependencies (bash 4+, GNU getopt, make
4+) required by `scripts/lib.sh`. When any script sources `lib.sh`, it
checks for these dependencies and fails if they're missing.

This caused consistent failures in the `test-go-pg (macos-latest)` job
in `nightly-gauntlet.yaml`, which didn't have the GNU tools setup that
`ci.yaml` had. Commit 9a417df ("ci: add retry logic for Go module
operations") added a macOS GNU tools step to `ci.yaml`, but
`nightly-gauntlet.yaml` was not updated.

This PR adds a reusable `setup-gnu-tools` action and uses it
consistently across all workflows with macOS jobs, replacing the inline
brew install steps.

Closes https://github.com/coder/internal/issues/1133

.github/workflows/release.yaml

@@ -78,14 +78,8 @@ jobs:
       - name: Fetch git tags
         run: git fetch --tags --force
 
-      - name: Setup build tools
-        run: |
-          brew install bash gnu-getopt make
-          {
-            echo "$(brew --prefix bash)/bin"
-            echo "$(brew --prefix gnu-getopt)/bin"
-            echo "$(brew --prefix make)/libexec/gnubin"
-          } >> "$GITHUB_PATH"
+      - name: Setup GNU tools (macOS)
+        uses: ./.github/actions/setup-gnu-tools
 
       - name: Switch XCode Version
         uses: maxim-lobanov/setup-xcode@60606e260d2fc5762a71e64e74b2174e8ea3c8bd # v1.6.0