Update dependency build to v4

Author: renovate[bot]

generator/pubspec.yaml

@@ -6,7 +6,7 @@ environment:
   sdk: '>=2.7.0 <3.0.0'
 
 dependencies:
-  build: ^1.2.2
+  build: ^4.0.0
   source_gen: ^0.9.5
   code_builder: ^3.2.1
   dart_style: ^1.3.3