PR check: add explicit setup-go step to unset-env check

Go has been removed from the PATH in the most recent Mac runner images rollouts.

Author: angelapwen

.github/workflows/__unset-environment.yml

@@ -10,6 +10,9 @@ steps:
   - uses: ./../action/.github/actions/setup-swift
     with:
       codeql-path: ${{ steps.init.outputs.codeql-path }}
+  - uses: actions/setup-go@v5
+    with:
+      go-version: '>=1.21.0'
   - name: Build code
     shell: bash
     # Disable Kotlin analysis while it's incompatible with Kotlin 1.8, until we find a