misc: use a copy of sources when temporarily feeding a new source (#23935)

wcs/qommon/substitution.py

@@ -74,10 +74,11 @@ class Substitutions(object):
             yield
             return
 
+        orig_sources, self.sources = self.sources, self.sources[:]
         self.sources.append(source)
         self.invalidate_cache()
         yield
-        self.sources.remove(source)
+        self.sources = orig_sources
         self.invalidate_cache()
 
     @classmethod