Update the flutter script's locking mechanism and follow_links (#57590)

Update the flutter and dart scripts' locking mechanism and follow_links function to be more robust and support more platforms.

This adds support for using mkdir as a fallback if the system doesn't have flock instead of using shlock, since shlock doesn't work on shared filesystems.

It also fixes a problem in the follow_links function where it failed when the link resolved to the root directory.