Add explicit sync after downgrading OSTree install

The OSTree tests are occasionally failing at boot when downgrading the OSTree commit:

[  197.241328] workqueue: Failed to create a rescuer kthread for wq "ext4-rsv-conversion": -EINTR
[  197.241378] EXT4-fs: failed to create workqueue
[  197.255226] EXT4-fs (mmcblk0p1): mount failed
[[0;1;31mFAILED[0m] Failed to mount [0;1;39mboot.mount[0m - /boot.
[[0;1;38;5;185mDEPEND[0m] Dependency failed for [0;1;39mloca…s.target[0m - Local File Systems.
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Adding a sync to the process has been shown to reduce, but not resolve the issue. Add a sync in order to improve the outcome of testing until a full fix can be found.

Signed-off-by: Martyn Welch martyn.welch@collabora.com