[CI] Allow for running arbitrary scripts

Summary:
This feature already exists but requires to specify a script file to
run. It is more versatile to let the user write the script directly in
the YAML file, since it can always call an external script as needed.

Test Plan:

./contrib/teamcity/build-configurations.py lint-circular-dependencies
./contrib/teamcity/build-configurations.py build-osx

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8821