Ignore things in .gitignore in tidy

- Switch from `walkdir` to `ignore`. This required various changes to
  make `skip` thread-safe.
- Ignore `build` anywhere in the source tree, not just at the top-level.
  We support this in bootstrap, we should support it in tidy too.

As a nice side benefit, this also makes tidy a bit faster.

Before:
```
; hyperfine -i '"/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"'
Benchmark 1: "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"
  Time (mean ± σ):      1.080 s ±  0.008 s    [User: 2.616 s, System: 3.243 s]
  Range (min … max):    1.069 s …  1.099 s    10 runs
```

After:
```
; hyperfine '"/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"'
Benchmark 1: "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"
  Time (mean ± σ):     705.0 ms ±   1.4 ms    [User: 3179.1 ms, System: 1517.5 ms]
  Range (min … max):   702.3 ms … 706.9 ms    10 runs
```

.gitignore

@@ -41,7 +41,7 @@ no_llvm_build
 /inst/
 /llvm/
 /mingw-build/
-/build/
+build/
 /build-rust-analyzer/
 /dist/
 /unicode-downloads