ci: install debian-archive-keyring for mkosi bootstrap

mkosi's apt-get (inside the mkosi workspace) couldn't verify Debian's
InRelease signatures because the act_runner's Ubuntu base image ships
Ubuntu's keyring, not Debian's. Adding `debian-archive-keyring` to the
apt install list exposes /usr/share/keyrings/debian-archive-keyring.gpg
which debootstrap and apt need for the bookworm repos.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -33,7 +33,8 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install -y --no-install-recommends \
-            mkosi debootstrap squashfs-tools dosfstools systemd-ukify
+            mkosi debootstrap squashfs-tools dosfstools systemd-ukify \
+            debian-archive-keyring
 
       - name: Install templ
         run: go install github.com/a-h/templ/cmd/templ@v0.3.1001