[GUIX] Git ignore the GUIX build files

Summary: We don't want the guix output/var files to be tracked nor included in the source package.

Test Plan:
Run a guix build, then ./contrib/guix/guix-clean
Check git status report no file changed, but the output and var dirs are not erased.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13774