ensure we call EasyBlock.patch_step for postinstallpatches

[branfosj]

(created using eb --new-pr)

fixes #4040

The bundle easyblock patch_step is a no-op:

    def patch_step(self):
        """Patch step must be a no-op for bundle, since there are no top-level sources/patches."""
        pass

and this causes postinstallpatches to fail when used in any easyconfig is a bundle or that derives from that easyblock.

By calling EasyBlock.patch_step() directly we make sure we are calling the correct function and we can use postinstallpatches in a PythonBundle easyconfig.

[boegel]

lgtm