Fix Dataflow job file validation on Windows

[ulofiai]

Fixes #18734.

Use File.isAbsolute() when validating dataflowJobFile and templateLocation. This accepts Windows drive-letter and UNC paths while preserving the existing Unix absolute-path and gs:// checks.

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[jrmccluskey]

Are there any existing test cases that are disabled on windows that we could use to validate the fix here? Or can we refactor a bit to make unit testing more viable?