tidy: Fix false positives with absolute repo paths in `pal.rs` `check()`

Fixes the bug:
1. git clone https://github.com/rust-lang/rust.git rust-improve-tests
2. cd rust-improve-tests
3. ./x test tidy

Expected:
No tidy errors found

Actual:
```
thread 'pal (library)' (837175) panicked at src/tools/tidy/src/pal.rs:100:5:
assertion failed: saw_target_arch
```

Since the git checkout dir contains the word "tests", the `pal.rs`
`check()` used to erroneously ignore all paths.