ENH: split CI per Python via variant.use_keys on noarch cuda-python output

The CFEP-25 noarch:python recipe references only ${{ python_min }}, so
rattler-build does not include `python` in that output's variant.
Combined with the non-noarch `cuda-bindings` output (which does use
`python`), conda-smithy's _collapse_subpackage_variants takes the
intersection of used loop vars across outputs, drops `python`, and
collapses all Python variants into a single .ci_support file per
platform — serializing what used to be 6 parallel CI jobs.

Adding `build.variant.use_keys: [python]` to the noarch output forces
`python` back into its variant, restoring the per-Python matrix. The
redundant identical noarch builds dedupe on upload via hash collision.

Mirrors the same pattern used in conda-forge/root-feedstock for its
noarch:generic root_cxx_standard output.

Author: leofang

recipe/recipe.yaml

@@ -31,6 +31,14 @@ outputs:
       number: ${{ number }}
       # skip: not linux64
       noarch: python
+      variant:
+        # Ensures conda-smithy splits CI jobs per Python version. Without
+        # this, the noarch output drops `python` from the intersection of
+        # used loop vars (since CFEP-25 uses `python_min`), collapsing all
+        # Python variants into a single serialized job. The redundant noarch
+        # builds dedupe on upload via hash collision.
+        use_keys:
+          - python
       script:
         env:
           SUBDIR: ${{ subdir }}