bump atc
bump atc

[finishes #114631577]

Submodule src/github.com/concourse/atc 7b51199..6242488:
  > fix fetching image resources on tagged workers

concourse/concourse,cjcjameson/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,cjcjameson/concourse,cjcjameson/concourse