add User-Agent env var

[deer-wmde]

https://phabricator.wikimedia.org/T397052

• Adds a custom User-Agent string to Blazegraph requests when running federated queries
• Includes the wdqs image version it depends on, see https://docs.docker.com/reference/dockerfile/#understand-how-arg-and-from-interact
• fixes outdated github cache action version number

bash-4.4# env | grep USER
USER_AGENT=Wikibase.Cloud Query Service (0.3.135-wmde.13); https://github.com/wbstack/queryservice

• for verification, skaffold this image and try inspecting platform-nginx logs while using the Query Service
  • kubectl logs -f deployments/platform-nginx