This is useful for rust-lang/rust to allow setting a sysroot that's
*only* for build scripts, different from the regular sysroot passed in
RUSTFLAGS (since cargo doesn't apply RUSTFLAGS to build scripts or
proc-macros).
That said, the exact motivation is not particularly important: this
fixes a regression from
5907e9155e (r1060215684).
Note that only RUSTFLAGS is tested in the new integration test; passing
--sysroot through `clippy-driver` never worked as far as I can tell, and
no one is using it, so I didn't fix it here.
321c530fad