CI: ensure pure python code when testing forks.

The idea here is that random error on Windows are
*maybe* caused by  the bindings to C made here.
Earlier we have seen that calling the routines
using C (swig) in xtgeo may have issues when
doing forking/multiporccessing.

examples/multiprocess_surfs.py

@@ -58,7 +58,7 @@ def _get_regsurff(i):
     sfile = TESTFILE
 
     logger.info("File is %s", sfile)
-    rf = xtgeo.surface_from_file(sfile)
+    rf = xtgeo.surface_from_file(sfile, fformat="irap_binary", engine="python")
     logger.info("End %s", i)
     return rf