style cleanup in GHC easyconfigs

for https://github.com/hpcugent/easybuild-easyconfigs/pull/1211
this is the style we maintain in easyconfigs...

and actually, since these paths seem to be identical across versions, the `sanity_check_paths` should be defined in the `GHC` easyblock instead, to avoid repeating them in the easyconfigs over and over...