fix non-portable GNU find(1)-ism

GNU find(1)'s -not operator is not POSIX compliant.  Use ! operator
instead in order to work with other find(1) implementations.

script/build

@@ -28,7 +28,7 @@ setup_gopath() {
 }
 
 find_source_files() {
-  find . -maxdepth 2 -name '*.go' -not -name '*_test.go' "$@"
+  find . -maxdepth 2 -name '*.go' '!' -name '*_test.go' "$@"
 }
 
 count_changed_files() {