CI: fuzz: Run `cargo fmt` and `cargo clippy`

[drinkcat]

Looks like we have no CI test that runs cargo fmt and cargo clippy in fuzz directory.

cd fuzz && cargo fmt does work, so we could add that reasonably easily.

But cd fuzz && cargo clippy doesn't, as it looks like cfg(fuzzing) is not set in that case.

error[E0603]: module `number` is private
  --> fuzz_targets/fuzz_seq_parse_number.rs:9:13
   |
9  | use uu_seq::number::PreciseNumber;
   |             ^^^^^^ private module
   |
note: the module `number` is defined here
  --> /home/drinkcat/dev/coreutils/coreutils/src/uu/seq/src/seq.rs:24:1
   |
24 | mod number;
   | ^^^^^^^^^^

Fixed what was possible to fix in #7717, but we should find a way to automate this.

[aaron-ang]

Hi @drinkcat, I also faced this issue in one of my PRs #8513 when adding a new project dependency. fuzz should be part of the project workspace, and it is marked as a TODO in Cargo.toml. What is the rationale for introducing cfg(fuzzing) in the first place? Can we use a feature flag instead? Happy to take this up and work on it.