Run all workspace tests in CI (x86-64 Linux)

[drinkcat]

In #7383, we noticed that CI is not running all tests. For example, tests for package uucore are not being run.

I believe the right solution is to run the tests with the --workspace flag:

cargo test --workspace

I did some experiments in #7386, where I first tried to add --workspace unconditionally to the cargo test command.

windows/macos/linux-musl builds are broken: we could try to fix this later (I'll probably file other issues).

Even x86_64-unknown-linux-gnu builds are built using cross. First, the selinux dependency was not met. This could be fixed by adding a Cross.toml file that pulls in libselinux1-dev (see this example). But even then, the problem is that ghcr.io/cross-rs/x86_64-unknown-linux-gnu:0.2.5 is ancient (Ubuntu 16.04),
so the provided libclang is too old, and the build fails with:

  A `libclang` function was called that is not supported by the loaded `libclang` instance.

      called function = `clang_getTranslationUnitTargetInfo`
      loaded `libclang` instance = 3.8.x

  The minimum `libclang` requirement for this particular function can be found here:
  https://docs.rs/clang-sys/latest/clang_sys/clang_getTranslationUnitTargetInfo/index.html

  Instructions for installing `libclang` can be found here:
  https://rust-lang.github.io/rust-bindgen/requirements.html

My current idea is to add a new job to the matrix that uses cargo directly (no cross). But that currently fails with:

error[E0658]: lint reasons are experimental
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/selinux-0.5.0/src/errors.rs:144:24
    |
144 |     #[allow(dead_code, reason = "used by unit tests")]
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #54503 <https://github.com/rust-lang/rust/issues/54503> for more information

For more information about this error, try `rustc --explain E0658`.

[drinkcat]

#7394 fixes the error above.

Looks like the uucore tests are being run, at the very least:

     Running unittests src/lib/lib.rs (target/x86_64-unknown-linux-gnu/debug/deps/uucore-b0572bc14cb6090b)
running 142 tests

So I think this is good enough. I'll clean up #7386.