[FIXED JENKINS-46597] Further handling of serializability issues

[abayer]

• JENKINS issue(s):
  • JENKINS-46597
• Description:
  • Specifically: the checkout step returns a TreeMap, so let's make sure we never have a variable assigned directly to that return value. Probably want to change checkout to return a HashMap in the
    future, but we'll cross that bridge later. We can't actually test this normally because our core and workflow-scm-step versions are too old to have that functionality, but I did verify it by hand
  • Also updated workflow-cps dependency so that we can be sure BooleanClosureWrapper serialization issues won't crop up, letting us write more things as .every or .any.
• Documentation changes:
  • n/a
• Users/aliases to notify:
  • @reviewbybees

[ghost]

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[rsandell]

🐝