Follow CARGO_BUILD_TARGET env as target

[Vaider7]

Currently there's no way to override compilation target expect setting rust-analyzer.cargo.target option. That's a little bit annoying to modify config files every time you need LSP support for a target different from your host platform.

So I would like to propose that rust analyzer must follow CARGO_BUILD_TARGET if it is present as build target. I'm eager to implement it if the idea is approved