modify py312 migrator not to override our channel_sources

.ci_support/migrations/python312.yaml

@@ -19,6 +19,7 @@ __migrator:
     paused: false
     longterm: True
     pr_limit: 60
+    use_local: True
     max_solver_attempts: 5  # this will make the bot retry "not solvable" stuff 5 times
     exclude:
       # this shouldn't attempt to modify the python feedstocks
@@ -34,7 +35,8 @@ __migrator:
 python:
   - 3.12.* *_cpython
 channel_sources:
-  - conda-forge/label/python_rc,conda-forge
+  - conda-forge/label/python_rc,conda-forge  # [unix]
+  - conda-forge/label/python_rc,conda-forge/label/llvm_rc,conda-forge/label/meson_dev,conda-forge  # [win]
 # additional entries to add for zip_keys
 numpy:
   - 1.26