configs/releng/airootfs/etc/systemd/system/pacman-init.service: don't hardcode the keyrings

If the keyring is not specified, pacman-key will simply use all keyrings from /usr/share/pacman/keyrings/.

Fixes #133.
3 jobs for pacman-init.service-populate in 34 minutes and 47 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Lint
passed #22234
shellcheck

00:00:11

 
  Build
passed #22235
fast-single-thread
build: [baseline]

00:14:41

passed #22236
fast-single-thread
build: [releng]

00:34:30