Use 0o666 permissions for flock files instead of 0o777 (#16845)
This removes executable permissions while retaining global read / global
write.
It's been suggested we should use 0o644 instead, dropping the global
write permissions (i.e., just the owner can write), but since we're
taking an exclusive lock I don't think that would work and we'd regress
the issue that was solved by updating the permissions. I think we'll
need to revisit the locking scheme if that's the goal, but regardless,
this seems like a net improvement.
