[GUIX] Let cmake use the environment variable for (C|CXX|LD)FLAGS

Summary:
And remove the redundant flags.
This has the main benefit of erasing the dynamic loader so the binaries can run outside of the guix environment.

Test Plan:
Run the guix build.
Copy the binaries to some other machine and check it runs.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13778