Tests: install the container's tools from our own mirror, and try again

An install test failed 13 seconds in: a public Arch mirror dropped two
signature files while the test container was installing QEMU. The three
test containers now ask mirror.omarchy.org first, with the public
mirrors after it, and try up to three times.

.github/workflows/unit-test.yml

@@ -46,7 +46,14 @@ jobs:
               sleep 1
             done
             echo "systemd: $state"
-            pacman -Syu --noconfirm --needed python parted diffutils jq >/dev/null
+            # From our own Arch mirror first, and a second and third try: a
+            # mirror that drops one file must not fail the tests.
+            sed -i "1i Server = https://mirror.omarchy.org/\$repo/os/\$arch" /etc/pacman.d/mirrorlist
+            for attempt in 1 2 3; do
+              pacman -Syu --noconfirm --needed python parted diffutils jq >/dev/null && break
+              [ "$attempt" -lt 3 ] || { echo "::error::the test container could not install its tools"; exit 1; }
+              sleep 15
+            done
             useradd -m tester
             cp -r /src /home/tester/omarchy-iso
             chown -R tester /home/tester/omarchy-iso