Impossible to submit Google Dataflow job from Windows machine

Got exception when trying to submit Dataflow job from java application running on Windows machine.
> java.lang.IllegalArgumentException: Location must be local or on Cloud Storage, got C:\Users\myuser\AppData\Local\Temp\dataflow-job341260414234234.json.
>  Here:

[https://github.com/apache/beam/blob/67f6cf2c7786e339758854efb1cf1230909e4f1d/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/DataflowRunner.java#L746](https://github.com/apache/beam/blob/67f6cf2c7786e339758854efb1cf1230909e4f1d/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/DataflowRunner.java#L746)

following check is performed:  fileLocation.startsWith("/") || fileLocation.startsWith("gs://"),

which prohibits usage of any Windows-style file names.

Workaround is to provide explicit file location on GCS:

dataflowPipelineOptions.setDataflowJobFile("gs://mybucket/dataflowjob.pb");

Imported from Jira [BEAM-4380](https://issues.apache.org/jira/browse/BEAM-4380). Original Jira may contain additional context.
Reported by: kardashov.

Start at runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/DataflowRunner.java around line 746 and inspect the file-location check for local and Cloud Storage paths. Reproduce submission from a Windows machine, then verify that the Windows temporary path is accepted while the existing gs:// workaround and local-path behavior remain valid.

Written by the indexing model from the issue text.