fix: add missing #[serial] to test_musicbrainz_dump_url_default (#274)

The test calls env::remove_var but lacked #[serial], allowing it to
race with the serial test_musicbrainz_dump_url_from_env that sets the
same env var — causing flaky CI failures.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: SimplicityGuy

extractor/src/tests/config_tests.rs

@@ -196,6 +196,7 @@ fn test_from_env_secret_file_not_found() {
 }
 
 #[test]
+#[serial]
 fn test_musicbrainz_dump_url_default() {
     unsafe {
         std::env::remove_var("MUSICBRAINZ_DUMP_URL");