This fixes a race condition in test/integration.shard/cache_test.dart which allowed the test to fail if the machine took a very long time to acquire a lock or launch a process.
This changes the "Waiting for another flutter command to release the startup lock..." message output so that it appears on stderr instead of stdout. When it appears on stdout, it can mess up collection of the output. For instance, if you run flutter --version --machine and you're expecting JSON output, then you'll get non-JSON output even though the lock is released and you eventually would get what you're asking for.
