Consider correct dependent when skipping build-time deps

We skip build-time deps when installing from bottles. However, the
current logic only considers the root, rather than the actual dependent
formula. Given

  A (bottled)
  |__B (not bottled)
     |__C (build-time)

C will be pruned from the effective dependency tree of A. This is wrong,
because C is required in order to build B.

Fix this by examining the current dependent rather than the root.

Fixes Homebrew/homebrew#17356.

Library/Homebrew/formula_installer.rb

@@ -136,7 +136,7 @@ class FormulaInstaller
           if dep.optional? || dep.recommended?
             Dependency.prune unless dependent.build.with?(dep.name)
           elsif dep.build?
-            Dependency.prune if pour_bottle?
+            Dependency.prune if install_bottle?(dependent)
           end
 
           if f.build.universal?