Generated rust-project.json files now include sysroot paths

[UebelAndre]

Fixes #1576

Special thanks to @snowp!

[UebelAndre]

This is an alternate implementation of #1580 which doesn't add any rust-analyzer fields to the core rust_toolchain. The somewhat downside of this implementation though is that unlike #1580 I'm updating rust-analyzer toolchain to begin to contain it's own sysroot which is defined separately from rust_toolchain. I ended up on this implementation due to the fact that rust_toolchain does not expose the raw files it ingests and instead, creates a unique sysroot for actions here. To my knowledge, the rust-analyzer sysroot is always expected to run on the host so using anything in execroot could lead to unexpected behavior. So to have a simple location that won't change out from under rust-analyzer in the output_user_root for a workspace, rust-analyzer now directly depends on a rustc binary. If this is not necessary I think it'd be great to clean up but the thing I like most about this approach is that rust_toolchain is left alone.

[snowp]

Thanks for taking this over the finish line!