tests: 'guix-gc.sh' passes even when 'out' or 'drv' are defined as env vars.

This fixes a test failure exhibited by
fb17a899 whereby assignments to 'out' in
guix-gc.sh would go to the 'out' environment variable, when it exists,
which in turn prevents garbage collection of $out.

* tests/guix-gc.sh: Add 'unset' invocations.