[qodelicious]

Fixes #1154

--timings was declared as --timings: string in the cargo externs, so nushell expects a value after it and cargo build --timings fails with "Missing flag argument". In cargo the value is optional (--timings[=<FMTS>]) and the plain flag is how it's normally used.

Changed it to a switch in all 7 places (build, check, doc, test, bench, install, clippy).

Tested with a stub cargo that just echoes its args

> use cargo-completions.nu *
> cargo build --timings
cargo got: build --timings

Before this it errored with nu::parser::missing_flag_param. It also stops cargo test --timings --release from treating --release as the timings value.

[fdncred]

This could also be a oneof<string, bool> but we can try it the way it is. Thanks