gobject-introspection: Don't hardcode /nix/store

If no config.nix.storeDir has been set, don't fall back to "/nix/store"
but use builtins.storeDir instead so we always should end up with the
correct store path no matter whether config.nix.storeDir has been set.

Thanks to @lethalman for pointing this out.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>