Allow appending to the PATH in environment sections

You should be able to append to the path in the environment sections of a pipeline. Scripted pipelines support this through PATH+WHATEVER syntax in the `withEnv` step.

Attempting to use PATH = "${PATH}:..." at a top level results in this error being thrown "groovy.lang.MissingPropertyException: No such property: PATH", and attempting to use PATH+WHATEVER results in "(PATH + GO) is a binary expression, but it should be a variable expression"