Update only_one to use flock (advisory lock) instead of pidfile

[wjordan]

The implementation of only_one.rb (#only_one_running?) reads a pidfile and (if it exists) sends a kill 0 to see if a previously-started process is still running. If not, the current process writes its PID to that file and deletes it with an at_exit hook on shutdown.

This script had two (known remaining) issues, both related to the at_exit hook deleting the pidfile on shutdown:

1. Forked processes also call the at_exit hook when they exit, causing the file to be deleted before the process actually exited;
2. A process can hang in between the at_exit hook and the process actually shutting down, causing the file to be deleted before the process actually exited.

These issues could be dealt with by fixing them individually (see #35069 for a fix to 1), or by removing the at_exit entirely and depending only on the kill 0 rather than the file-existence to determine if another process is still running.

This PR offers an approach using advisory file locks (see flock(2)) via File#flock, which ends up being simpler than the approach using pidfiles and kill 0.

Testing story

We didn't have existing test coverage for this script, so I added a simple test OnlyOneTest covering basic behavior using fork that fails on staging but passes with either this PR or #35069.

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[Hamms]

This approach looks great! Love how close we're able to get to a bare-metal bash solution here. Love the test, too!