Add python_rc label alongside python_dev for Python 3.15 marker dep

The conda-forge 3.15 alpha builds depend on the _python_rc marker package,
which is only published on the python_rc label. Include both
conda-forge/label/python_dev (interpreter) and conda-forge/label/python_rc
(_python_rc marker) so the solve succeeds.

Author: atalman

.github/scripts/validate_binaries.sh

@@ -28,14 +28,15 @@ get_python_config() {
             CONDA_EXTRA_PARAM=" -c conda-forge/label/python_rc -c conda-forge"
             ;;
         3.15t)
-            # 3.15 is pre-release: conda-forge ships it under the python_dev
-            # label (alphas), not python_rc. Pin the latest available alpha.
+            # 3.15 is pre-release: conda-forge ships the interpreter under the
+            # python_dev label (alphas), but the build depends on the _python_rc
+            # marker that lives on the python_rc label, so both are required.
             PYTHON_V=3.15.0a8
-            CONDA_EXTRA_PARAM=" python-freethreading -c conda-forge/label/python_dev -c conda-forge"
+            CONDA_EXTRA_PARAM=" python-freethreading -c conda-forge/label/python_dev -c conda-forge/label/python_rc -c conda-forge"
             ;;
         3.15)
             PYTHON_V=3.15.0a8
-            CONDA_EXTRA_PARAM=" -c conda-forge/label/python_dev -c conda-forge"
+            CONDA_EXTRA_PARAM=" -c conda-forge/label/python_dev -c conda-forge/label/python_rc -c conda-forge"
             ;;
         *)
             PYTHON_V=${MATRIX_PYTHON_VERSION}