[CI] Use script paths relative to the project root in the configuration

Summary:
This diff makes the script path relative to the project root instead of
the contrib/teamcity directory, so it is easier to call a script from
anywhere in the source tree.

Test Plan:

./contrib/teamcity/build-configurations.py gitian-linux

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7534