[Infra] Update check_copyright.sh to ignore root third_party directories (#13403)

Author: ncooke3

scripts/check_copyright.sh

@@ -24,6 +24,7 @@ options=(
 list=$(git grep "${options[@]}" -- \
     '*.'{c,cc,cmake,h,js,m,mm,py,rb,sh,swift} \
     CMakeLists.txt '**/CMakeLists.txt' \
+    ':(exclude)third_party/**' \
     ':(exclude)**/third_party/**')
 
 # Allow copyrights before 2020 without LLC.