Fix build-iso: copy localrepo to /tmp and patch pacman.conf for CI

The local-aur repo in pacman.conf points to /home/alvin/alvinarch/localrepo
which only exists on the dev machine. In CI, copy the tracked .pkg.tar.zst
files to /tmp/localrepo and sed the path before building.

.gitea/workflows/build-iso.yaml

@@ -15,6 +15,12 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Setup local AUR repo
+        run: |
+          mkdir -p /tmp/localrepo
+          cp localrepo/*.pkg.tar.zst localrepo/local-aur.db* /tmp/localrepo/
+          sed -i 's|file:///home/alvin/alvinarch/localrepo|file:///tmp/localrepo|' pacman.conf
+
       - name: Install archiso and dependencies
         run: |
           pacman -Syu --noconfirm archiso jq