formula_cellar_checks: check_non_libraries only for new formulae

This check is overly opinionated and leads to audit failures no one
cares to fix, so let's only run it for new formulae.

Library/Homebrew/formula_cellar_checks.rb

@@ -157,7 +157,7 @@ module FormulaCellarChecks
     problem_if_output(check_manpages)
     problem_if_output(check_infopages)
     problem_if_output(check_jars)
-    problem_if_output(check_non_libraries)
+    problem_if_output(check_non_libraries) if @new_formula
     problem_if_output(check_non_executables(formula.bin))
     problem_if_output(check_generic_executables(formula.bin))
     problem_if_output(check_non_executables(formula.sbin))