message: Fix formats_source test requiring rustfmt.
diff: use cargo_test_support::compare::assert_ui;
  use cargo_test_support::prelude::*;
  -use cargo_test_support::Project;
  +use cargo_test_support::{process, Project};
  use cargo_test_support::curr_dir;
  -#[cargo_test(requires_rustfmt)]
  +#[cargo_test]
  fn formats_source() {
  + // This cannot use `requires_rustfmt` because rustfmt is not available in
  + // the rust-lang/rust environment. Additionally, if running cargo without
  + // rustup (but with rustup installed), this test also fails due to HOME
  + // preventing the proxy from choosing a toolchain.
  + if let Err(e) = process("rustfmt").arg("-V").exec_with_output() {
  + eprintln!("skipping test, rustfmt not available:\n{e:?}");
  + return;
  + }
  let project = Project::from_template(curr_dir!().join("in"));
  let project_root = &project.root();
  