ostree-prepare-root.service: add OnFailureJobMode=isolate

This is stronger than the default (`replace`) because it tells systemd
to *stop everything* and go to `emergency.target`. In other codebases,
this has definitely helped me with the problem of "systemd keeps going
even after a failure".

Likely addresses ostreedev#3219.

See also e.g. coreos/ignition-dracut@3d2e165.

src/boot/ostree-prepare-root.service

@@ -19,11 +19,13 @@ Documentation=man:ostree(1)
 DefaultDependencies=no
 ConditionKernelCommandLine=ostree
 ConditionPathExists=/etc/initrd-release
-OnFailure=emergency.target
 After=sysroot.mount
 Requires=sysroot.mount
 Before=initrd-root-fs.target
 
+OnFailure=emergency.target
+OnFailureJobMode=isolate
+
 [Service]
 Type=oneshot
 ExecStart=/usr/lib/ostree/ostree-prepare-root /sysroot