Unit tests: the container needs diff and jq

The offline mirror index test compares databases with diff and the
nightly test reads JSON with jq. Neither is in the Arch base image, so
both tests failed in CI while passing on a workstation.

.github/workflows/unit-test.yml

@@ -46,7 +46,7 @@ jobs:
               sleep 1
             done
             echo "systemd: $state"
-            pacman -Syu --noconfirm --needed python parted >/dev/null
+            pacman -Syu --noconfirm --needed python parted diffutils jq >/dev/null
             useradd -m tester
             cp -r /src /home/tester/omarchy-iso
             chown -R tester /home/tester/omarchy-iso