Reverting my .pub-cache change to figure out why it's still failing. (#13355)

* Revert "Add tests."

This reverts commit 31bad961ff2220a2708917ff8f748fed7efa9b51.

* Revert "Use .pub-cache from Flutter root, if it exists. (#13248)"

This reverts commit 72d6bcc3f7555c5a242f46cb09484f6c12e86f39.