tests/test_cosalib_meta.py: don't deepcopy GenericBuildMeta

It seems like jsonschema in Python 3.12 (in Fedora 39) changed some
implementation details to now use a data structure that doesn't support
deep copying:

    TypeError: cannot pickle 'rpds.HashTrieMap' object

Instead of doing a deep copy, just create a new object from scratch with
the same inputs. The `w` variable might've been for that; it's currently
unused and is an exact copy of `m`.

tests/test_cosalib_meta.py

@@ -143,13 +143,13 @@ def get_aws(x, key="path"):
             m = meta.GenericBuildMeta(_create_test_files(tmpdir, meta_data=td),
                                       '1.2.3')
 
+            # create working copies
             w = meta.GenericBuildMeta(_create_test_files(tmpdir, meta_data=td),
                                       '1.2.3')
-            # create working copies
             if x is None:
-                x = copy.deepcopy(m)
+                x = w
             else:
-                y = copy.deepcopy(m)
+                y = w
 
             # add the stamp
             m.write()