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.

crates/project-model/src/env.rs

@@ -62,11 +62,10 @@ pub(crate) fn inject_rustc_tool_env(env: &mut Env, cargo_name: &str, kind: Targe
 
 pub(crate) fn cargo_config_env(
     manifest: &ManifestPath,
-    extra_env: &FxHashMap<String, String>,
+    extra_env: &FxHashMap<String, Option<String>>,
     sysroot: &Sysroot,
 ) -> Env {
-    let mut cargo_config = sysroot.tool(Tool::Cargo, manifest.parent());
-    cargo_config.envs(extra_env);
+    let mut cargo_config = sysroot.tool(Tool::Cargo, manifest.parent(), extra_env);
     cargo_config
         .args(["-Z", "unstable-options", "config", "get", "env"])
         .env("RUSTC_BOOTSTRAP", "1");