rapidcheck: Use stock installation, multiple outputs
We had neglected to nice `RC_INSTALL_ALL_EXTRAS`, which will take care
of installing the "extras" for us.
Now that they are in a standard directory, we don't need to smuggle in
the extra `-I` for Nix's tests either.
(cherry picked from commit ab8cf01579ce0eba0202b0c84a71fd54ccfe4fc2)
(cherry picked from commit 0acb4d6350b81f2d8276f1dd22a7ae7cec515a37)
