message: Fix bug that would have caused every crate build to fail
  docs.rs needs build tests :(
  ```
  2020/09/04 23:06:51 [INFO] rustwide::cmd: [stderr] error: Unrecognized option: 'disable-per-crate-search--extern-html-root-url'
  ```
diff: @@ -582,7 +582,9 @@ impl RustwideBuilder {
  let mut env_vars = metadata.environment_variables();
  let rustdoc_flags = env_vars.entry("RUSTDOCFLAGS").or_default();
  - rustdoc_flags.push_str(" --static-root-path / --cap-lints warn --disable-per-crate-search");
  + // WARNING: this *must* end with a space or it will cause rustdoc to give an error
  + rustdoc_flags
  + .push_str(" --static-root-path / --cap-lints warn --disable-per-crate-search ");
  rustdoc_flags.push_str(&rustdoc_flags_extras.join(" "));
  let mut command = build
  