Imbue nogil migrator with allowlist (like osx-arm64)

[h-vetinari]

This idea came up in conda-forge/conda-forge-pinning-feedstock#6673, and is a good way to start migrating for nogil, without turning on the full firehose of python-dependent packages. By filtering the graph on an allowlist in the pinning repo, we can make conscious choices which packages to include, e.g. initially only those that are known to be compatible.

[zklaus]

We added a general allowlist feature with #4858, and are applying this to the nogil case in h-vetinari/conda-forge-pinning-feedstock#1 and h-vetinari/conda-forge-pinning-feedstock#2.