Add purism user to systemd-journal group

This allows it to read the system journal as well which is useful
for debugging.
4 jobs for systemd-journal in 3 minutes and 56 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Package
passed #360402
librem5
package-pureos-byzantium

00:00:50

passed #360403
librem5:arm64
package-pureos-byzantium:arm64

00:02:48

 
  Test Package
passed #360404
librem5
autopkgtest-pureos-byzantium-package

00:03:01

passed #360405
librem5
lintian-pureos-byzantium-package

00:00:46