Display lockfile path in very verbose mode when blocking

[TanmayArya-1p]

What does this PR try to resolve?

Addresses #15094

When waiting on a lock, the path to the lockfile is displayed when -vv is passed.

tanmay@victus:~/foo$ cargo build -vv
    Blocking waiting for file lock on artifact directory (/home/tanmay/foo/target/debug/.cargo-lock)

[TanmayArya-1p]

I'm unsure if a test is required for this change and where it should go if required. I was thinking maybe in testsuite/concurrent.rs.

[weihanglo]

I'm unsure if a test is required for this change and where it should go if required. I was thinking maybe in testsuite/concurrent.rs.

Either there or here in tests/testsuite/cache_lock.rs. We do have some tests around that but pretty brutal:

cargo/tests/testsuite/cache_lock.rs

Lines 72 to 99 in a374e43

	/// Helper to verify that acquiring the given mode would block.
	///
	/// Always call `thread_wait_timeout` on the result.
	#[must_use]
	fn verify_lock_would_block(mode: CacheLockMode) -> JoinHandle<()> {
	let root = paths::root();
	// Spawn a thread that will block on the lock.
	let thread = std::thread::spawn(move || {
	let gctx = GlobalContextBuilder::new().root(root).build();
	let locker2 = CacheLocker::new();
	let lock2 = locker2.lock(&gctx, mode).unwrap();
	assert!(locker2.is_locked(mode));
	drop(lock2);
	});
	// Verify that it blocked.
	retry(100, || {
	if let Ok(s) = std::fs::read_to_string(paths::root().join("shell.out")) {
	if s.trim().starts_with("Blocking waiting for file lock on") {
	return Some(());
	} else {
	eprintln!("unexpected output: {s}");
	// Try again, it might have been partially written.
	}
	}
	None
	});
	thread
	}

cargo/tests/testsuite/global_cache_tracker.rs

Lines 1308 to 1319 in a374e43

	// Wait for clean to finish.
	let thread = std::thread::spawn(|| clean_child.wait_with_output().unwrap());
	let output = thread_wait_timeout(100, thread);
	assert!(output.status.success());
	// Validate the output of the clean.
	execs()
	.with_stderr_data(str![[r#"
	[BLOCKING] waiting for file lock on package cache mutation
	[REMOVED] [FILE_NUM] files, [FILE_SIZE]B total
	"#]])
	.run_output(&output);

[rustbot]

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[weihanglo]

Thanks!

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