Figure out an automated way to validate against new bootlibs

Probably yes; I forgot this time. I wonder if there's some way to automate this (we already have the appropriate inputs, but they can only be used with `workflow_dispatch`). Maybe a label, but then we need some way to pass it the appropriate inputs.

_Originally posted by @geekosaur in https://github.com/haskell/cabal/issues/11127#issuecomment-3152600600_