Deduplicate Cargo in the RLS on CI

[alexcrichton]

Once #52919 lands we'll have updated the RLS which updates racer, and racer has recently picked up a dependency on the cargo crate. Unfortunately though this is a cargo crate from crates.io, which differs from the src/tools/cargo crate in-tree. This means that when we build the RLS it's linking against two optimized versions of Cargo, which not only takes up a good deal of space but also takes up a good deal of build time.

In talking with @nrc on IRC one solution here would be to avoid using racer's cargo feature. The RLS already depends on cargo-the-crate and should be able to provide the functionality necessary for racer, for example implementing racer's ProjectModelProvider trait

[alexcrichton]

In fixing this we should add a tidy check which ensures that in rust-lang/rust's workspace there's only one entry in the lock file for the cargo crate

[nrc]

cc racer-rust/racer#914

[mati865]

cc #52072