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/actions/setup-gnu-tools/action.yaml

@@ -0,0 +1,18 @@
+name: "Setup GNU tools (macOS)"
+description: |
+  Installs GNU versions of bash, getopt, and make on macOS runners.
+  Required because lib.sh needs bash 4+, GNU getopt, and make 4+.
+  This is a no-op on non-macOS runners.
+runs:
+  using: "composite"
+  steps:
+    - name: Setup GNU tools (macOS)
+      if: runner.os == 'macOS'
+      shell: bash
+      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"