pipeline: bind mount the osbuild module for the stages

The dnf stage wants to import `osbuild.sources` but currently the
osbuild module is not available in the stages. Apply the same hack
done in the Assembler also in for the stages, i.e. bind mount the
osbuild module to the stages/osbuild.

osbuild/pipeline.py

@@ -80,11 +80,20 @@ class Stage:
 
             sources_dir = f"{libdir}/sources" if libdir else "/usr/lib/osbuild/sources"
 
+            ro_binds = []
+            if not libdir:
+                osbuild_module_path = os.path.dirname(importlib.util.find_spec('osbuild').origin)
+                # This is a temporary workaround, once we have a common way to include osbuild in the
+                # buildroot we should remove this because it includes code from the host in the buildroot thus
+                # violating our effort of reproducibility.
+                ro_binds.append(f"{osbuild_module_path}:/run/osbuild/lib/stages/osbuild")
+
             with API(f"{build_root.api}/osbuild", args, interactive) as api, \
                 sources.SourcesServer(f"{build_root.api}/sources", sources_dir, source_options, secrets):
                 r = build_root.run(
                     [f"/run/osbuild/lib/stages/{self.name}"],
                     binds=[f"{tree}:/run/osbuild/tree"],
+                    readonly_binds=ro_binds,
                     stdin=subprocess.DEVNULL,
                 )
                 res = BuildResult(self, r.returncode, api.output)