Kokoro change detection (and then running tests) is broken

Currently each handwritten directory has a .kokoro/trampoline_v2.sh script which is run, and which tries to detect changes within that directory.

Unfortunately, it looks like there are misaligned expectations of the current directory and `$PROJECT_ROOT`.

- Each script is looking for changes within `handwritten/xyz` when the current directory is already `handwritten/xyz`, so the tests are never triggering
- If the test somehow *does* trigger, `.trampolinerc` tries to find `.kokoro` under `handwritten/xyz/handwritten/xyz` which also fails.

#8509 is a first pass at addressing this, but probably needs more work.

As the tests for handwritten code are never triggering, this issue doesn't block Librarian migration.