Ensure licenses excluded file list has newline at EOF (#38354)

Author: zanderso

ci/licenses_golden/excluded_files

@@ -2911,4 +2911,4 @@
 ../../../third_party/zlib/google/zip_reader_unittest.cc
 ../../../third_party/zlib/google/zip_unittest.cc
 ../../../third_party/zlib/patches/README
-../../../tools
+../../../tools

ci/licenses_golden/tool_signature

@@ -1,2 +1,2 @@
-Signature: 2aaa7d1662adb75aac43db23f4bd5d97
+Signature: e9bc01b7e51cb2185dc056dffd7ff351
 

tools/licenses/lib/main.dart

@@ -2121,8 +2121,11 @@ Future<void> main(List<String> arguments) async {
         system.exit(1);
       }
       // write to disk the list of files we did _not_ cover, so it's easier to catch in diffs
+      final String excluded = (_RepositoryDirectory._excluded.map(
+        (fs.IoNode node) => node.fullName,
+      ).toSet().toList()..sort()).join('\n');
       system.File(path.join(argResults['out'] as String, 'excluded_files')).writeAsStringSync(
-        (_RepositoryDirectory._excluded.map((fs.IoNode node) => node.fullName).toSet().toList()..sort()).join('\n'),
+        '$excluded\n',
       );
     }
   } catch (e, stack) {