[guix] Fix guix-clean on CI

Summary: It is not possible to pass a bash array as an environment variable, so the extra excludes are not properly expanded and cause git clean to fail silently.

Test Plan:
Create some in tree directory, then:

./contrib/teamcity/build-configurations.py guix-osx

Check the directlry is removed.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15741