VRT Derived Bands: Allow mul and sum to apply constant factor to a single band

[dbaston]

No description provided.

[coveralls]

coverage: 70.59% (+0.06%) from 70.531%
when pulling 22383dc on dbaston:vrt-pixelfn-constant-factor
into 9bffc4c on OSGeo:master.

[rouault]

@dbaston There's a test failure on Windows: https://github.com/dbaston/gdal/actions/runs/14440899536/job/40490611506 . I suspect that must be related to Trace = f'open("{tmp_path}/num_sources_skip_true.txt", "wt").write(str(len(in_ar)))'. On Windows '{tmp_path}' will expand to a string with backslashes. But when reinjected in ret, {Trace} will be evaluated in a Python context at runtime, and so those backslashes should be escaped.