Rollup merge of #133712 - RalfJung:rust_analyzer_settings, r=jieyouxu

rust_analyzer_settings: force use of 'nightly' toolchain

The cranelift folder contains a rust-toolchain file. That means when RA opens `compiler/rustc_codegen_cranelift/Cargo.toml`, it will try to use that toolchain or fail. (Maybe that toolchain gets auto-installed for others? On my system, it just fails, but I also run vscodium in a sandbox.)

However, it shouldn't be necessary to use more than one toolchain for the rustc workspace. So we can set the `RUSTUP_TOOLCHAIN` variable on the server side to force the same toolchain to be used everywhere.

Suggested by `@ChayimFriedman2` in https://github.com/rust-lang/rust-analyzer/issues/18585.