o/s/backend: restore properly when EnsureKernelDriversTree fails #432

Job	Run time
cache-build-deps	1m 2s
snap-builds	0s
static-checks (1.18)	3m 21s
static-checks (latest/stable)	4m 22s
branch-static-checks	0s
unit-tests-special (1.18, snapd_debug)	4m 51s
unit-tests-special (1.18, withbootassetstesting)	5m 33s
unit-tests-cross-distro (fedora:39)	6m 13s
unit-tests-special (1.18, nosecboot)	5m 3s
unit-tests-cross-distro (opensuse/tumbleweed)	5m 18s
unit-tests (1.18, normal)	5m 4s
unit-tests (latest/stable, normal)	5m 28s
unit-tests-special (1.18, faultinject)	5m 14s
unit-tests-special (1.18, race)	9m 4s
unit-tests-special (latest/stable, snapd_debug)	5m 7s
unit-tests-special (latest/stable, withbootassetstesting)	5m 5s
unit-tests-special (latest/stable, nosecboot)	5m 48s
unit-tests-special (latest/stable, faultinject)	5m 20s
unit-tests-special (latest/stable, race)	9m 37s
${{ matrix.group }}	0s
code-coverage	7s
	1h 31m 37s

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