fix: Set RUSTUP_TOOLCHAIN when loading sysroot workspace

[Veykril]

No description provided.

[weihanglo]

This was mentioned in rust-lang/rust#136779 (comment), though I had a hard time figuring out what the bug this tried to fix here is? Would you mind sharing a bit more on this?

[Veykril]

I believe the issue here is that we run cargo metadata on the sysroot library source but prior to this PR we did not set the RUSTUP_TOOLCHAIN env var for that invocation, and as such, if the cargo we invoke here is the rustup proxy then that might pick a different toolchain cargo than the one we want to use for current workspace being analyzed.