this makes three changes:
- all callsites of `toolchain::command` are changed to use
`command(path, extra_env)`, instead of manually adding the env after
the fact.
- all `map<str, str>` are changed to `map<str, option<str>>`.
- `command` checks for None and calls `env_remove` if so.
this caught several places where environment variables weren't being
propagated:
- when running `rustc --print=target-libdir`
- when running `cargo rustc -- --print=target-spec-json`
- when running the custom DiscoverLinkedProjects config. I *think* this
is for use with non-cargo build systems, so I didn't change it.
These flags were added to help rust-analyzer integrate with repos
requiring non-Cargo invocations. The consensus is that having two
independent settings are no longer needed. This change removes
`invocationLocation` in favor of `invocationStrategy` and changes
the internal representation of `InvocationStrategy::Once` to hold
the workspace root.
