[Storage] Increase the parallelism for upload in `MOUNT_CACHED`

[Michaelvll]

skypilot/sky/data/mounting_utils.py

Lines 491 to 497 in 64f419c

	# '--transfers 1' guarantees the files written at the local mount point
	# to be uploaded to the backend storage in the order of creation.
	# '--vfs-cache-poll-interval' specifies the frequency of how often
	# rclone checks the local mount point for stale objects in cache.
	# '--vfs-write-back' defines the time to write files on remote storage
	# after last use of the file in local mountpoint.
	'--transfers 1 --vfs-cache-poll-interval 10s --vfs-write-back 1s '

We set the transfer parallelism to 1 for the guaranteed order, but it makes the upload slow.