update: a failing hook without run_update fails the update

Since a65f07c ran each hook in a subshell, a hook that does its work as
it's sourced (12-migrations, 15-services, 20-aur, 30-flatpak and others)
had its failure lost: the run_update check after source left the
subshell's status at 0, so update said "Done." and exited 0. The smoketest's
40-failing-migration caught it. update now exits the subshell with the
sourced hook's status.

Also, the smoketest scenarios that check whether a migration ran look for
"Running migration " so install.sh's "Running migrations..." heading
doesn't match (11-refresh failed on it).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

smoketest/scenarios/32-new-migration.sh

@@ -10,10 +10,10 @@ scenario() {
 
   cd_update
   assert_status 0
-  assert_output "Running migration"
+  assert_output "Running migration "
   cd_update
   assert_status 0
-  refute_output "Running migration"
+  refute_output "Running migration "
 
   assert_vm "the migration ran exactly once" '[ "$(cat ~/smoketest-migration)" = ran ]'
   assert_vm "the migration is recorded" '