Remove hardcoded paths in flock behavior

[esigler]

Since I'm running/testing this on an OS that doesn't have /var/lock, I'd like to have all the tests actually pass. There's still a flapper somewhere in the flock code where sometimes the tests pass and sometimes they don't, but this at least cleans up the pathing & makes it more platform agnostic.

[ranjib]

Flock support was added to prohibit same script running on the same machine as two different processes (similar to runonce), hence an flock with tempdir might be suboptimal. we can make the path argument as a mandatory argument if we cant find a platform neutral lock path

[theckman]

I'm okay with /tmp/blender-(id).lock or something. Realistically, you shouldn't go in there deleting files willy-nilly. And if you do, you deserve to have things break.

[esigler]

Reverted my default directory change, kept where paths are hardcoded in tests & released lock cleanup.