ci: only run nightlies on pushes to main (#965)

* just pip install whl?

* nightlies only on main

.github/workflows/extremes.yml

@@ -91,7 +91,8 @@ jobs:
       - name: Run doctests
         run: pytest narwhals --doctest-modules
 
-  pandas-nightly-and-dask:
+  nightlies:
+    if: github.event_name == 'push'
     strategy:
       matrix:
         python-version: ["3.11"]
@@ -118,7 +119,7 @@ jobs:
         run: |
           kaggle kernels output "marcogorelli/variable-brink-glacier"
       - name: install-polars
-        run: uv pip install *.whl --system
+        run: pip install *.whl --system
       - name: install-reqs
         run: uv pip install --upgrade tox virtualenv setuptools pip -r requirements-dev.txt --system
       - name: uninstall pyarrow