[guix] Don't look for dependencies when building the source package

Summary:
Not that it hurts, but it is useless and takes time for no reason.
Also remove the RPATH option that is not needed as nothing is linked in this step.

Test Plan: Run the GUIX build

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15470