Fix for plugin failing when it is run on a slave agent with error java.lang.IllegalStateException: Jenkins.instance is missing

[Siddharth-Srinivas]

Fixes made:

• Removed credential reading in the callable where the agent does not have access to the Jenkins instance.
• Read credentials to be used for the setup in the Agent and pass it down to the callable.
• PATH setting on remote agents fail gracefully and show a guidance message now to set up manually

Testing done

Verified this works on a Jenkins setup with remote agent. This is the use case where the plugin was failing

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[vinay-mummadi-digicert]

reviewed