Add `clippy::unconditional_recursion` to `./x clippy ci`

The clippy lint catches some things that rustc's equivalent builtin lint
does not, for example rust-lang/rust#146940:

    error: function cannot return without recursing
        --> library/std/src/path.rs:3428:5
         |
    3428 | /     fn eq(&self, other: &String) -> bool {
    3429 | |         self == &*other
    3430 | |     }
         | |_____^
         |
    note: recursive call site
        --> library/std/src/path.rs:3429:9
         |
    3429 |         self == &*other
         |         ^^^^^^^^^^^^^^^
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unconditional_recursion
         = note: requested on the command line with `-D clippy::unconditional-recursion`

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