nixpkgs' alertmanager uses DynamicUser, so the `alertmanager` user only
exists while the service is running. The matrix-alertmanager-urlfile
secret is owned by that user; at boot sops-install-secrets cannot resolve
it and rejects the whole manifest, leaving /run/secrets empty (no ssh
host key, yggdrasil key, tor key, ...). Deploys masked it because the
dynamic user was resolvable via nss-systemd at switch time.
Create a static alertmanager user/group and disable DynamicUser.
kurogeek