message: Fix tests on Linux/MacOS
  The tests were failing with error:
  "could not determine the current user, please set $USER"
diff: @@ -663,6 +663,7 @@ fn formats_source() {
  fs::write(&paths::root().join("rustfmt.toml"), "tab_spaces = 2").unwrap();
  cargo_process("init --lib")
  + .env("USER", "foo")
  .with_stderr("[CREATED] library package")
  .run();
  @@ -682,6 +683,7 @@ mod tests {
  #[cargo_test]
  fn ignores_failure_to_format_source() {
  cargo_process("init --lib")
  + .env("USER", "foo")
  .env("PATH", "") // pretend that `rustfmt` is missing
  .with_stderr("[CREATED] library package")
  .run();
  