In TMT configuration, use PACKIT_SOURCE_BRANCH environment variable for multi-repo management

[mulkieran]

With this variable available to our TMT configuration, we should be able to do much the same as we do in https://github.com/stratis-storage/github-actions/tree/master/stratisd-modify . This will allow the tests that do not directly make use of installed stratisd executables to compile and run when they are dependent on changes in any of our Rust libraries.