See gist.

SSHAgentStepExecution probably needs to be capturing EnvVars from the step context and passing that to the launcher in initRemoteAgent. And ExecRemoteAgentFactory and ExecRemoteAgent would need to pass the EnvVars to every command.

Just never came up before because ssh-agent would not normally care about environment variables other than SSH_AGENT_PID (which should not normally be initially set on the node anyway); $PATH / %Path% is of course relevant when launching the command, but this was only ever tested on Linux, with ssh-agent in /usr/bin.

Probable workaround: preconfigure the %Path% of the computer with ssh-agent available by default (or do that in the agent launcher), rather than relying on withEnv.