Fix a problem with the flutter_tools/test/dart/pub_get_test.dart test. (#13354)

It turns out that when a PR build clones the flutter repo, it puts it into a directory called "flutter", and when the "real" build clones the flutter repo, it puts it into a directory called "build". This allowed the tests to succeed in my PR, but fail when I committed it.

This fixes the test so it doesn't depend on the cloned directory name.