Add a new configuration settings to set env vars when running cargo, rustc, etc. commands: cargo.extraEnv and checkOnSave.extraEnv
It can be extremely useful to be able to set environment variables when rust-analyzer is running various cargo or rustc commands (such as `cargo check`, `cargo --print cfg` or `cargo metadata`): users may want to set custom `RUSTFLAGS`, change `PATH` to use a custom toolchain or set a different `CARGO_HOME`.
There is the existing `server.extraEnv` setting that allows env vars to be set when the rust-analyzer server is launched, but using this as the recommended mechanism to also configure cargo/rust has some drawbacks:
- It convolutes configuring the rust-analyzer server with configuring cargo/rustc (one may want to change the `PATH` for cargo/rustc without affecting the rust-analyzer server).
- The name `server.extraEnv` doesn't indicate that cargo/rustc will be affected but renaming it to `cargo.extraEnv` doesn't indicate that the rust-analyzer server would be affected.
- To make the setting useful, it needs to be dynamically reloaded without requiring that the entire extension is reloaded. It might be possible to do this, but it would require the client communicating to the server what the overwritten env vars were at first launch, which isn't easy to do.
This change adds two new configuration settings: `cargo.extraEnv` and `checkOnSave.extraEnv` that can be used to change the environment for the rust-analyzer server after launch (thus affecting any process that rust-analyzer invokes) and the `cargo check` command respectively. `cargo.extraEnv` supports dynamic changes by keeping track of the pre-change values of environment variables, thus it can undo changes made previously before applying the new configuration (and then requesting a workspace reload).
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