build: set SOURCE_DATE_EPOCH

Library/Homebrew/build.rb

@@ -144,6 +144,9 @@ class Build
         # which is not known until after the formula has been staged.
         ENV["HOMEBREW_FORMULA_PREFIX"] = formula.prefix
 
+        # https://reproducible-builds.org/docs/source-date-epoch/
+        ENV["SOURCE_DATE_EPOCH"] = formula.source_modified_time.to_i.to_s
+
         formula.patch
 
         if args.git?