filesystem: Ignore blkid cache

Sometimes, `blkid` will incorrectly return no information about a block
device, even if it exists and has a valid filesystem. This causes the
*filesystem* module to fail if *force=no*. Instructing `blkid` to use
`/dev/null` as a cache file will force it to rescan the block device on
each run, making results more consistent.

Signed-off-by: Dustin C. Hatch <admiralnemo@gmail.com>