[MNT] differential testing of cython based estimators (#5206)

This PR upgrades the `test-cython-estimators` that runs the test suite
on `cython` based estimators to the status of the other test suite runs,
in that it should run tests only on estimator classes whose modules have
changed with respect to `main`.

This is achieved by the presence of the `main` branch that is now also
avaliable to this CI element - without, it simply always tests all
estimators.

.github/workflows/test.yml

@@ -141,6 +141,11 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v4
+
+      - run: git remote set-branches origin 'main'
+
+      - run: git fetch --depth 1
+
       - name: Set up Python
         uses: actions/setup-python@v4
         with: