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/e2e.yml

@@ -37,6 +37,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y --no-install-recommends \
             mkosi debootstrap squashfs-tools systemd-ukify \
+            debian-archive-keyring \
             qemu-system-x86 qemu-utils \
             dnsmasq iperf3 ipxe-qemu