Nightly: keep downloaded packages between runs

A nightly downloads the packages of the ISO's offline mirror, a few
gigabytes, and the test container's own, and would download most of them
again the next night. On Namespace both directories live on the runner
profile's cache volume. The build already prunes from its cache what an
ISO no longer needs. The step is skipped, not failed, on a runner
without a cache volume.

.github/workflows/nightly-build.yml

@@ -68,6 +68,18 @@ jobs:
 
       - uses: docker/setup-buildx-action@37fe631027851001ddb9b187196cc803df7f5f0e # v4
 
+      # What a build downloads and would download again tomorrow: the packages
+      # the ISO's offline mirror is made of (the build prunes the ones it no
+      # longer needs) and the test container's own. Skipped, not failed, on a
+      # runner without a cache volume.
+      - name: Package cache (Namespace)
+        continue-on-error: true
+        uses: namespacelabs/nscloud-cache-action@c5f8dab7560444c4bf8dbc64f1b203431873c547 # v1.6.1
+        with:
+          path: |
+            ~/.cache/omarchy
+            ~/.cache/omarchy-ci
+
       - name: Build the edge ISO
         run: NO_BOOT_OFFER=1 ./bin/omarchy-iso-make --edge
 
@@ -76,7 +88,9 @@ jobs:
       - name: Install unattended, boot the installed system, seal it
         run: |
           sudo chmod 0666 /dev/kvm
+          mkdir -p ~/.cache/omarchy-ci/pacman
           docker run --rm --device /dev/kvm -v "$PWD:/w" -w /w \
+            -v "$HOME/.cache/omarchy-ci/pacman:/var/cache/pacman/pkg" \
             -e OMARCHY_INTEGRATION_DISK_CACHE=none \
             archlinux:latest bash -euo pipefail -c '
               # pacman 7 fails every sync on a kernel without Landlock, which