ci: fix Host key verification failed

- missing SSH known_hosts in deploy pipeline
- SSH tries to use Trust-On-First-Use (TOFU) interactively to add a new
  host key
- verbose SSH logs:
debug1: Server host key: ssh-ed25519 SHA256:1bbksDNYBWSh/rIFP7MMfs557kWn1dM64bpXdnfBE5E
debug1: read_passphrase: can't open /dev/tty: No such device or address
- deploy-rs uses nix, which uses SSH which doesn't use the environment variable
HOME, but rather /etc/passwd to find a user's HOME

.drone.yml

@@ -28,16 +28,14 @@ steps:
       PRIVATE_SSH_KEY:
         from_secret: ci_private_ssh_key
       SSH_HOST_KEY: "80.244.242.4 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIE7XTCHfX6ta8EtkdOcZLnpdhMmXDfTebVMs4NC8JEPj"
+      # SSH uses HOME from /etc/passwd, not from the environment, so override it
+      NIX_SSHOPTS: "-o UserKnownHostsFile=$$HOME/.ssh/known_hosts -i $$HOME/.ssh/id_ed25519"
     commands:
-      - "mkdir ~/.ssh && chmod 700 ~/.ssh"
-      - echo "$$PRIVATE_SSH_KEY" > ~/.ssh/id_ed25519 && chmod 600 ~/.ssh/id_ed25519
-      - echo "$$SSH_HOST_KEY" > ~/.ssh/known_hosts
-      - echo DEBUG env
-      - env
-      - echo DEBUG ls -alh ~/.ssh
-      - ls -alh ~/.ssh
-      - 'echo DEBUG: Using NIX_FLAGS: $NIX_FLAGS'
-      - nix $$NIX_FLAGS develop --command deploy --magic-rollback false --skip-checks --targets '.#host_001_momo_koeln' --ssh-opts='-v' -- --impure
+      - mkdir $$HOME/.ssh && chmod 700 $$HOME/.ssh
+      - echo "$$PRIVATE_SSH_KEY" > $$HOME/.ssh/id_ed25519 && chmod 600 $$HOME/.ssh/id_ed25519
+      - echo "$$SSH_HOST_KEY" > $$HOME/.ssh/known_hosts
+      - "echo DEBUG: Using NIX_FLAGS: $$NIX_FLAGS"
+      - nix $$NIX_FLAGS develop --command deploy --magic-rollback false --skip-checks --targets '.#host_001_momo_koeln'
 
 ---
 kind: pipeline
@@ -163,6 +161,6 @@ volumes:
 
 ---
 kind: signature
-hmac: d10ac7912c87547d54d9883f456d5fbc5302fa9ca80941ed9a9b93080e32ef88
+hmac: c7083fb6372539aee0c22490f08252ec310de8e92f6d5b7d58872ffc649de660
 
 ...