xfs_scrub_all.timer: don't run if /var/lib/xfsprogs is readonly

xfs_scrub_all.timer: don't run if /var/lib/xfsprogs is readonly

The xfs_scrub_all program wants to write a state file into the package
state dir to keep track of how recently it performed a media scan.
Don't allow the systemd timer to run if that path isn't writable.

Cc: linux-xfs@vger.kernel.org # v6.10.0
Fixes: 267ae610a3d90f ("xfs_scrub_all: enable periodic file data scrubs automatically")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>