Auto merge of #14207 - tomokinat:master, r=lnicola
Respect $CARGO_HOME when looking up toolchains.
Some people set `$CARGO_HOME` to a location other than `~/.cargo` (`$XDG_DATA_DIR/cargo`, in my case), and I'd be a little nicer if the rust-analyzer extension and server respect that value when looking up toolchains, instead of having us configure all of `$CARGO`, `$RUSTC` ... manually.
The new implementation still defaults to `~/.cargo` if `$CARGO_HOME` is unset, pretty much like cargo itself does (as documented in https://doc.rust-lang.org/cargo/guide/cargo-home.html), so the change is backwards compatible for most people except those who has configured `$CARGO_HOME` explicitly.
I considered using https://crates.io/crates/home as suggested by https://doc.rust-lang.org/cargo/guide/cargo-home.html, but decided to put int on hold because i) we need mirror impl in node, ii) I thought the consistency matters more and iii) the new implementation shouldn't be worse than the current one (i.e. switching to `home` improvement is rather orthogonal and could be done in another PR). If you have any directions on this, please let me know.
