Autoharness include/exclude pattern does not support whitespace

See https://github.com/model-checking/verify-rust-std/actions/runs/14673784039/job/41186052310?pr=334: when trying to use something like `--include-pattern "num::<impl i8>::wrapping_sh"` 
our splitting of `RUSTFLAGS` on whitespace in `base_rustc_flags` falls over.