SNAPCRAFT_PART_INSTALL doesn't expand in build-environment assignments

[林博仁(Buo-ren, Lin)]

The following assignment doesn't work:

  ```yaml
  parts:
    _part_name_:
      build-environment:
      - PYTHONPATH: $SNAPCRAFT_PART_INSTALL/lib/python3.6
  ```

It'll expand to `/lib/python3.6` in the `override-build` scriptlet.

Refer https://forum.snapcraft.io/t/problems-with-the-parts-part-name-build-environment-property/10642 for discussion.