When running GIMP in the build environment (even before it's installed), we
don't want to "fix" paths. We had the case in particular on the macOS CI where
the install PREFIX was a parent directory of the build directory and therefore
we were "fixing" some perfectly good constructed directories (set by meson) into
non-existing folder paths.

Additionally this codepath should only run when ENABLE_RELOCATABLE_RESOURCES is
set (even though this alone would not have fixed our CI issue because the macOS
build is relocatable).

Finally I am updating the gimp-data repository so that libraries are properly
found from the (now correct thanks to this commit) paths set by meson when
running gimp-console from within the build directory.