twoliter: allow relative project path

[webern]

Issue number:

Closes #279

Description of changes:

Make it work.

Testing done:

This was broken, now it works.

cargo run --package twoliter \
  --bin twoliter -- build kit extra-3-kit \
  --project-path ./tests/projects/local-kit/Twoliter.toml --arch x86_64

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.