allow using `null` to unset an environment variable

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.

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