Roll buildroot to 6669621484b649b9355fb2dd37950daf3d2ac65b (#18652)

flutter/buildroot@6669621484b649b9355fb2dd37950daf3d2ac65b

See flutter/buildroot#372
https://dart.googlesource.com/external/github.com/flutter/engine/+/c5d012900f7db0dc5dc9afb46f76d16b87915db0
diff --git a/commits.json b/commits.json
index 997be46..95537ef 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"389a59d5dfe9617f8f0da73611f7c8663d27f8e1",
+  "external/github.com/flutter/engine":"c5d012900f7db0dc5dc9afb46f76d16b87915db0",
   "external/github.com/flutter/flutter":"a1636b6fef2ebe7840ba6dc1b3f9d77095ef3b45",
   "sdk":"bbf34035dd984d7dedbcd8a53bc3c79a25b842e8"
 }
\ No newline at end of file