Package tests could read GHC version from `dist/setup-config`

Currently, when running the package tests with a non-default version of GHC, it's necessary to specify the `GHC` environment variable to point at whatever you want to be using.

It's probably more sensible to instead read the GHC version from the configuration information, where possible – note that we already rely on the inplace package conf being there, so this isn't too much more to ask.