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.

.github/workflows/__unset-environment.yml

@@ -79,6 +79,9 @@ jobs:
       - 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