Improvements to the fuzz-parser script (#11071)

## Summary

- Properly fix the race condition identified in
https://github.com/astral-sh/ruff/pull/11039. Instead of running the
version of Ruff we're testing by invoking `cargo run --release` on each
generated source file, we either (1) accept a path to an executable on
the command line or (2) if that's not specified, we run `cargo build
--release` once at the start and then invoke the executable found in
`target/release/ruff` directly.
- Now that the race condition is properly fixed, remove the workaround
for the race condition added in
https://github.com/astral-sh/ruff/pull/11039.
- Also allow users to pass in an executable to compare against for the
`--only-new-bugs` argument (previously it was hardcoded to always
compare against the version of Ruff installed into the Python
environment)
- Use `argparse.RawDescriptionHelpFormatter` as the formatter class
rather than `argparse.RawTextHelpFormatter`. This means that long help
texts for the individual arguments will be wrapped to a sensible width.
- On completion of the script, indicate success or failure of the script
overall by raising `SytemExit` with the appropriate exit code.
- Add myself as a codeowner for the script

.github/CODEOWNERS

@@ -12,3 +12,6 @@
 
 # flake8-pyi
 /crates/ruff_linter/src/rules/flake8_pyi/ @AlexWaygood
+
+# Script for fuzzing the parser
+/scripts/fuzz-parser/ @AlexWaygood