providers: set environment according to directory inside instances

[mr-cal]

• [X] Have you followed the guidelines for contributing?
• [X] Have you signed the CLA?
• [X] Have you successfully run make lint?
• [X] Have you successfully run pytest tests/unit?

---

When in a part's build directory inside an instance, the environment variables are set. This allows a user to manually build a part when debugging.

Builds on https://github.com/canonical/craft-parts/pull/303

TODO - add unit tests

(CRAFT-1412)