Do not rely on FilePathPickle

[jglick]

As of jenkinsci/workflow-durable-task-step-plugin#180, FilePath is not supposed to be directly serialized in program state, because it is tricky to resume a Java object referring to a live agent channel. WithMavenStepExecutionCallBack unfortunately did so, unlike say https://github.com/jenkinsci/credentials-binding-plugin/blob/8c080630bc65ef1eb1183d0e5cc179dc486122c0/src/main/java/org/jenkinsci/plugins/credentialsbinding/impl/UnbindableDir.java#L72 which would be used in

node(…) {
  withCredentials([file(…)]) {
    sh '…'
  }
}

This could mean that if an agent failed to reattach to the controller after a restart, a build could hang indefinitely with a message like

Still trying to load Looking for path named ‘/home/jenkins/agent/workspace/…@tmp/withMaven276c1fbe’ on computer named ‘…’

(And if you were using the new retry-with-conditions idiom, it could be aborted but not successfully resumed on a fresh agent.)

You can see in the virtual thread dump why the program does not load:

Program is not yet loaded
	SettableFuture@29c83fa0[status=SUCCESS, result=[org.jenkinsci.plugins.pipeline.modeldefinition.agent.impl.None@61b7b33e]] (complete)
	SettableFuture@5282f2e8[status=SUCCESS, result=[org.csanchez.jenkins.plugins.kubernetes.PodTemplate@3b66bb3b]] (complete)
	SettableFuture@533ba6c8[status=SUCCESS, result=[org.jenkinsci.plugins.pipeline.modeldefinition.agent.impl.None@fc9d1d2]] (complete)
	SettableFuture@4a997771[status=SUCCESS, result=[org.csanchez.jenkins.plugins.kubernetes.pipeline.PodTemplateStep@44a8a27a]] (complete)
	SettableFuture@4aab51c1[status=SUCCESS, result=[org.csanchez.jenkins.plugins.kubernetes.pipeline.KubernetesAgentErrorCondition@bad4e07]] (complete)
	SettableFuture@a0e82fc[status=SUCCESS, result=[org.jenkinsci.plugins.workflow.steps.SynchronousResumeNotSupportedErrorCondition@b344ab5]] (complete)
	SettableFuture@1b81274b[status=SUCCESS, result=[org.jenkinsci.plugins.workflow.support.steps.ExecutorStep@7abd8f4a]] (complete)
	SettableFuture@4daeb83[status=SUCCESS, result=[org.jenkinsci.plugins.pipeline.maven.publishers.GeneratedArtifactsPublisher@2d92784f]] (complete)
	Looking for path named ‘/home/jenkins/agent/workspace/…@tmp/withMaven276c1fbe’ on computer named ‘…’
	SettableFuture@3f3ba9c9[status=SUCCESS, result=[hudson.util.Secret@29eaa6bb]] (complete)

Filed jenkinsci/workflow-cps-plugin#755 in the course of identifying the withMaven step as the culprit.