Skip third_party/benchmark in the license source crawl (#3986)

Also regenerate sky_engine/LICENSE to include libtxt

sky/packages/sky_engine/LICENSE

@@ -9194,6 +9194,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 etc1
 observatory_pub_packages
 skia
+txt
 vulkan
 
 Apache License

tools/licenses/lib/main.dart

@@ -1846,6 +1846,7 @@ class RepositoryRootThirdPartyDirectory extends RepositoryGenericThirdPartyDirec
   @override
   bool shouldRecurse(fs.IoNode entry) {
     return entry.name != 'appurify-python' // only used by tests
+        && entry.name != 'benchmark' // only used by tests
         && entry.name != 'dart-sdk' // redundant with //engine/dart; https://github.com/flutter/flutter/issues/2618
         && entry.name != 'firebase' // only used by bots; https://github.com/flutter/flutter/issues/3722
         && entry.name != 'gyp' // build-time only