ci: escape registry vars inside remote deploy heredoc

Drone also blanks ${SECRET} in the <<'EOS' body before SSH; $$ so
the Pi shell expands the exported login env.

.drone.yml

@@ -135,7 +135,7 @@ steps:
           "$${SSH_USER}@$${SSH_HOST}" \
           "export REGISTRY_USERNAME='$${REGISTRY_USERNAME}' REGISTRY_PASSWORD='$${REGISTRY_PASSWORD}'; bash -s" <<'EOS'
         set -euo pipefail
-        echo "${REGISTRY_PASSWORD}" | docker login registry.zea.lt -u "${REGISTRY_USERNAME}" --password-stdin
+        echo "$${REGISTRY_PASSWORD}" | docker login registry.zea.lt -u "$${REGISTRY_USERNAME}" --password-stdin
         cd /home/miczek/homelab/tracker
         docker compose pull
         docker compose up -d --remove-orphans