hadrian: Call ghc-pkg recache after copying package database into bindist

The package.cache needs to have a later mod-time than all of the .conf
files. This invariant can be destroyed by `cp -r` and so we run `ghc-pkg
recache` to ensure the package database which is distributed is
consistent.

If you are installing a relocatable bindist, for example, on windows,
you should preserve mtimes by using cp -a or run ghc-pkg recache after
installing.