Don't assume 1 jar on classpath in portable jar creator

Right now we assume there is exactly 1 jar, the Spark/Flink job server jar, on the classpath. That assumption is not always correct. We should make this code more flexible, so it works even when there are other jars on the classpath. Perhaps by writing only the specific jar the PortablePipelineJarCreator class came from.

https://github.com/apache/beam/blob/32dd786f2d6cff02743421c0bb01bd3eea60dbbf/runners/java-job-service/src/main/java/org/apache/beam/runners/jobsubmission/PortablePipelineJarCreator.java#L142