uucore: enable process feature of rustix

[cakebaker]

This PR enables the process feature of rustix to fix an "unresolved import" error that currently shows up when running the pr tests:

$ cargo test --features=pr --no-default-features
   Compiling uucore v0.8.0 (/home/dho/projects/coreutils/src/uucore)
error[E0432]: unresolved import `rustix::process`
   --> src/uucore/src/lib/features/mode.rs:182:21
    |
182 |         use rustix::process::umask;
    |                     ^^^^^^^ could not find `process` in `rustix`
    |
note: found an item that was configured out
   --> /home/dho/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.4/src/lib.rs:269:9
    |
267 | #[cfg(feature = "process")]
    |       ------------------- the item is gated behind the `process` feature

[oech3]

Hmm... I considered that test CI for individual utils are dupe of full test, but we need
cargo check --tests --no-default-features --features uu_* on CI at least for.

[xtqqczze]

This seems to be a reoccurring issue with CI...

[sylvestre]

patches to detect this in the ci are welcome :