Vine: Add symlink mount option.

[dthain]

Proposed changes

Attaching large directory trees to the sandbox is expensive by default b/c the worker does a recursive hard-link of the entire directories. (This is to avoid the tasking being able to "tell" that the object is linked in.)

This PR adds a mount option allowing the object to be attached by a single symlink instead, if the user asks for it.

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Additional comments

Considering whether VINE_MOUNT_SYMLINK should be used by default in add_environment, which would cover most of the poncho use cases.