defaults: Set default systemd job stop timeout to 30 seconds

This avoids the phone getting stuck for a long time on shutdown,
trying to stop a misbehaving service. If a service doesn't stop
gracefully after 30 seconds, there's little chance it ever will.
5 jobs for stop-timeout in 4 minutes and 18 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #367588
librem5
shellcheck

00:00:07

 
  Package
passed #367589
librem5
package-pureos-byzantium

00:00:36

passed #367590
librem5:arm64
package-pureos-byzantium:arm64

00:02:04

 
  Test Package
passed #367591
librem5
autopkgtest-pureos-byzantium-package

00:03:31

passed #367592
librem5
lintian-pureos-byzantium-package

00:00:51