Create binstubs atomically.

[sigurdm]

Currently we write the binstub, and chmod it to be executable. There is a race here with concurrent invocations. (see flutter/flutter#144408)

Instead we could write it to a temp-location, chmod it and move it to PUB_CACHE/bin atomically.

[sigurdm]

Seems we already do this.

pub/lib/src/global_packages.dart

Lines 847 to 867 in ed20b45

	try {
	final tmpPath = p.join(tempDir, binStubPath);
	// Write this as the system encoding since the system is going to
	// execute it and it might contain non-ASCII characters in the
	// path names.
	writeTextFile(tmpPath, binstub, encoding: const SystemEncoding());
	if (Platform.isLinux || Platform.isMacOS) {
	// Make it executable.
	var result = Process.runSync('chmod', ['+x', tmpPath]);
	if (result.exitCode != 0) {
	// Couldn't make it executable so don't leave it laying around.
	fail('Could not make "$tmpPath" executable (exit code '
	'${result.exitCode}):\n${result.stderr}');
	}
	}
	File(tmpPath).renameSync(binStubPath);
	} finally {
	deleteEntry(tempDir);
	}

Was done in #3285