GNUmake: Fix Python Install (force) (#2655)

Local developers and cached CI installs ddi never install `pywarpx`
if and old version existed. The `--force` must be with us.

Author: ax3l

Source/Make.WarpX

@@ -235,7 +235,7 @@ endif
 
 installwarpx: libwarpx.$(PYDIM).so
 	mv libwarpx.$(PYDIM).so Python/pywarpx
-	cd Python; python setup.py install --with-libwarpx $(PYDIM) $(PYINSTALLOPTIONS)
+	cd Python; python setup.py install --force --with-libwarpx $(PYDIM) $(PYINSTALLOPTIONS)
 
 libwarpx.$(PYDIM).a: $(objForExecs)
 	@echo Making static library $@ ...