use mount from util-linux

busybox mount seems to have a limit on how much it can read from
/proc/mounts (3kb), meaning if you have a large entry in there preceding
the mount point you're remounting, it'll fail like so:

    mount: can't find /sys/fs/cgroup in /proc/mounts

using mount from util-linux works

Signed-off-by: Clara Fu <[email protected]>
Co-authored-by: Mark Huang <[email protected]>
Co-authored-by: Alex Suraci <[email protected]>

Merge pull request concourse#250 from john681611/cache-failover

Allow Build to continue if cache pull fails

remove dependence on user.Current in tests