🔧(helm) fix mail provisioning env

I forgot to use the new variable on the deployment.

src/helm/env.d/dev/values.desk.yaml.gotmpl

@@ -51,6 +51,10 @@ backend:
     POSTGRES_PASSWORD: pass
     REDIS_URL: redis://default:pass@redis-master:6379/1
     MAIL_PROVISIONING_API_URL: "http://host.docker.internal:8000"
+    MAIL_PROVISIONING_API_CREDENTIALS:
+      secretKeyRef:
+        name: backend
+        key: MAIL_PROVISIONING_API_CREDENTIALS
     SENTRY_DSN: "https://b72746c73d669421e7a8ccd3fab0fad2@sentry.incubateur.net/171"
   command:
     - "gunicorn"