Add an environment variable to python-sip that extends its search
directories for .sip files.

It seems that we cannot easily change the destination folder of these
files though, so this variable must be set on a per-package basis (and
non through search-path).

--- sip/sipbuild/builder.py	2023-03-22 09:06:37.588792878 +0100
+++ sip/sipbuild/builder.py	2023-03-22 09:10:35.830181134 +0100
@@ -254,6 +254,10 @@
                     os.path.join(project.target_dir,
                             project.get_bindings_dir()))
 
+            # Add extra bindings from environment for GNU Guix.
+            if 'SIP_INCLUDE_DIRS' in os.environ:
+                sip_include_dirs.extend(os.environ['SIP_INCLUDE_DIRS'].split(os.pathsep))
+
             # Generate the sip.h file for the shared sip module.
             copy_sip_h(abi_major_version, project.build_dir,
                     project.sip_module, version_info=project.version_info)