ci: Install `cargo-nextest` with `--locked` (#22984)

This PR makes it so we install `cargo-nextest` with `cargo install
cargo-nextest --locked` in CI.

According to the
[docs](https://nexte.st/docs/installation/from-source/), this is the
**only** supported way to install `cargo-nextest` when building from
source.

Release Notes:

- N/A