Fixes a gradle error where it was failing to find a plugin because of an absolute path in the .flutter-plugins file instead of a relative path.

I had originally removed this variable because I thought it was redundant with the projectDir, but apparently I was wrong about that (one resolves to a relative path, and one resolves to an absolute path).

This PR reverts that part of the change and reintroduces the (not really) redundant variable.