Clear $ENV{ACKRC} in t/config-finder.t

If the user already has an $ACKRC set, t/config-finder.t fails.

To repro:
    touch /tmp/ackrc ; ACKRC=/tmp/ackrc make test
Merge pull request #461 from sdt/bugfix/config-finder

Clear $ENV{ACKRC} in t/config-finder.t