message: Clear `RUSTDOCFLAGS` for all tests
diff: @@ -1723,6 +1723,7 @@ fn _process(t: &OsStr) -> cargo::util::ProcessBuilder {
  .env_remove("RUSTDOC")
  .env_remove("RUSTC_WRAPPER")
  .env_remove("RUSTFLAGS")
  + .env_remove("RUSTDOCFLAGS")
  .env_remove("XDG_CONFIG_HOME") // see #2345
  .env("GIT_CONFIG_NOSYSTEM", "1") // keep trying to sandbox ourselves
  .env_remove("EMAIL")
  