stages/copy: don't append path for input roots

The `parse_input` function should return the root for the input
only and must not include the path. Otherwise we will duplicate
it.

stages/org.osbuild.copy

@@ -83,7 +83,7 @@ def parse_input(url: ParseResult, args: Dict):
     if root is None:
         raise ValueError(f"Unknown input '{root}'")
 
-    return os.path.join(root, url.path.lstrip("/"))
+    return root
 
 
 def parse_location(location, args):