GStreamer 1.0: Fix write_filename for coincs

I accidentally reverted a py2-py3 change in the last merge with O4-dev:

I haven't checked for other issues with the merge. A before/after comparison with O4-dev for our gracedb upload code should highlight any other differences.

It caused the pipeline to break after a few hours when writing coincs.

Tests

I've had background (unseeded) runs going for 15 hours, and they've output coincs without any errors.