formula_installer: fix build/test reqs not being pruned correctly

Fixes #7618.

Library/Homebrew/formula_installer.rb

@@ -457,7 +457,6 @@ class FormulaInstaller
         keep_build_test ||= runtime_requirements.include?(req)
         keep_build_test ||= req.test? && include_test? && dependent == f
         keep_build_test ||= req.build? && !install_bottle_for_dependent
-        keep_build_test ||= (dep = formula_deps_map[dependent.name]) && !dep.build?
 
         if req.prune_from_option?(build)
           Requirement.prune
@@ -465,6 +464,8 @@ class FormulaInstaller
           Requirement.prune
         elsif (req.build? || req.test?) && !keep_build_test
           Requirement.prune
+        elsif (dep = formula_deps_map[dependent.name]) && dep.build?
+          Requirement.prune
         else
           unsatisfied_reqs[dependent] << req
         end