force-cube fails when using gdal 3.8.4 because output is redirected

[Florian Katerndahl]

With GDAL v. 3.8.4, the entire output printed to stderr if no file is given. As such, the check for valid resampling methods fails. Additionally, the output now spans two lines.
The seemingly straight forward solution is to explicitly call the help on gdalwarp which works with both GDAL 3.0.4 and the latest release and adjust the context in the grep command.

I implemented the fix and am currently checking if this indeed fixes the issue without creating obvious errors.

Kind regards,
Florian