Fix CI: test_pick_latest_release was silently hitting the real network

pick_download(v) was missing fetch=_fixture_fetch, so it fell through to
a real HTTP fetch of the actual micropython.org board page instead of
the test's fixture data. It happened to keep matching the hardcoded
"1.28.0" expectation until upstream cut a real 1.29.0 release, which
broke it here today -- unrelated to the doc-only previous commit that
CI caught it on. 1.4s -> 0.02s confirms it's offline now.

Author: bdbarnett

cli/tests/test_firmware_download.py

@@ -115,7 +115,7 @@ def test_pick_latest_release(self):
         cat = load_catalog(fetch=_fixture_fetch, force=True)
         v = find_variant("ESP32_GENERIC", catalog=cat)
         assert v is not None
-        chosen = pick_download(v)
+        chosen = pick_download(v, fetch=_fixture_fetch)
         self.assertEqual(chosen["channel"], "release")
         self.assertIn("1.28.0", chosen["version"])