Fix tools/luci/build.py script (#26895)

https://dart.googlesource.com/external/github.com/flutter/engine/+/99b752ed2c8f87bb1d13d89a7cee3fb9caa3b1f2
diff --git a/commits.json b/commits.json
index 643583c..d36d198 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"98253370f53d81e8e9128a8d90666771a74bdc68",
+  "external/github.com/flutter/engine":"99b752ed2c8f87bb1d13d89a7cee3fb9caa3b1f2",
   "external/github.com/flutter/flutter":"b30c721e4c3ad79fc474ac1f91dac5f2becc5938",
   "sdk":"0740566438c510835b87cad99f088c7302c5ed5d"
 }
\ No newline at end of file