Nightly: no package cache

The cache kept the ISO's packages between runs. A package that has gone
missing from the mirror would then still be found on the runner, and the
nightly would build and pass while a fresh machine could not. Every
package is fetched again each night: with the T2 repository asked on
GitHub first, the build is 3 to 4 minutes without a cache.

.github/workflows/nightly-build.yml

@@ -98,18 +98,9 @@ jobs:
       - uses: docker/setup-buildx-action@37fe631027851001ddb9b187196cc803df7f5f0e # v4
         if: matrix.channel == 'edge'
 
-      # 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
-
+      # No package cache, on purpose: every package the ISO is made of is
+      # fetched from the mirrors again each night, so one that has gone
+      # missing there fails this build instead of being served from a cache.
       - name: Build the edge ISO
         if: matrix.channel == 'edge'
         run: NO_BOOT_OFFER=1 ./bin/omarchy-iso-make --edge
@@ -136,9 +127,7 @@ 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