Switch legacy builds to manual git

Author: JonRowe

.github/workflows/ci.yml

@@ -133,7 +133,8 @@ jobs:
       JRUBY_OPTS: ${{ matrix.container.jruby_opts || '--dev' }}
       NO_COVERAGE: true
     steps:
-      - uses: actions/checkout@v3
+      - run: git clone https://github.com/rspec/rspec-support.git $GITHUB_WORKSPACE
+      - run: git reset --hard $GITHUB_HEAD_REF
       - run: ${{ matrix.container.pre }}
       - run: script/legacy_setup.sh
       - run: ${{ matrix.container.post }}