CFEP-09 migrations can't be updated

[isuruf]

If a migration file needs to be updated for some reason, but doesn't need to be rebuilt, then we have to change thousands of feedstocks.

For eg: to add pypy, we have to enforce cpython for existing builds which means a change in conda_build_config.yaml and the python38 migrator, but we can't enforce cpython builds in python38 migrator because that is in feedstocks.

cc @conda-forge/core

[CJ-Wright]

Is this because we're going to ad that cpython constraint in the metadata patch? We need all future builds to have that constraint, but we can't force rerenders on all packages?

[isuruf]

@CJ-Wright, yes. Also, some versions of python38.yaml have python 2.7 aarch64 enabled and some don't (later ones) and adding the cpython constraint gives builds with cpython constraint and one without due to some weird interactions.