ci: install systemd-boot for mkosi bootctl

mkosi Bootable=yes shells out to bootctl kernel-identify on the host,
which ships in the systemd-boot package on Ubuntu (not in systemd
itself). Without it, the live-image build fails at the end with
"bootctl: not found" after successfully installing all packages.

.gitea/workflows/e2e.yml

@@ -36,7 +36,8 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install -y --no-install-recommends \
-            mkosi debootstrap squashfs-tools systemd-ukify \
+            mkosi debootstrap squashfs-tools \
+            systemd-ukify systemd-boot \
             debian-archive-keyring \
             qemu-system-x86 qemu-utils \
             dnsmasq iperf3 ipxe-qemu