fix build cache configuration

[bcumming]

This PR does two things:

1. fix some bugs that stopped build cache working
2. refactor the mirror setup to perform validation of build cache information in the recipe stage

point 2 is important: we validate and generate data when creating the recipe, and the builder takes the validated information from the recipe and generates the build directory. You might want to look at doing the same for the other stages (bootstrap, spack_mirrors_yaml)

• generate the yaml output in the constructor for mirrors, not on demand during generation.
  This would bring the patch into line with the current recipe/builder separation of responsibilities (whether that is a good split is a different topic)

note that this breaks the unit tests. I ran out of time to figure out how they work, so I didn't fix them.