Reenable Hurd CI

Currently broken:

  error[E0609]: no field `st_fsid` on type `&stat64`
     --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/os/hurd/fs.rs:301:36
      |
  301 |         self.as_inner().as_inner().st_fsid as u64
      |                                    ^^^^^^^ unknown field
      |
  help: a field with a similar name exists
      |
  301 |         self.as_inner().as_inner().st_uid as u64
      |                                    ~~~~~~
  
  For more information about this error, try `rustc --explain E0609`.
  error: could not compile `std` (lib) due to 1 previous error
error: process didn't exit successfully: `/home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo check -Z build-std=std,panic_abort --target i686-unknown-hurd-gnu --manifest-path Cargo.toml --no-default-features` (exit status: 101)

Seems like an issue with Rust's std lib.

Start by reproducing the failure with the reported cargo check command for the i686-unknown-hurd-gnu target and inspect the Hurd standard-library error shown in the issue. Trace the repository's Hurd CI configuration and determine what must change for that check to pass. Done means the Hurd CI job is enabled again and completes successfully.