message: Clean some more env vars during tests
  Will be required to run tests as part of rust-lang/rust
diff: @@ -51,7 +51,9 @@ fn _process(t: &OsStr) -> cargo::util::ProcessBuilder {
  .env("HOME", support::paths::home())
  .env("CARGO_HOME", support::paths::home().join(".cargo"))
  .env("__CARGO_TEST_ROOT", support::paths::root())
  + .env_remove("__CARGO_DEFAULT_LIB_METADATA")
  .env_remove("RUSTC")
  + .env_remove("RUSTDOC")
  .env_remove("RUSTFLAGS")
  .env_remove("CARGO_INCREMENTAL")
  .env_remove("XDG_CONFIG_HOME") // see #2345
  