Make scons use stdenv only if immediate build dep

formula_installer will use stdenv if scons is anywhere in the formula's
recursive dependency list:

    https://github.com/Homebrew/legacy-homebrew/issues/40401#issuecomment-110066355

Having scons as a dependency should only require stdenv if it is an
immediate build dependency, as otherwise scons shouldn't be invoked.

Library/Homebrew/formula_installer.rb

@@ -562,7 +562,7 @@ class FormulaInstaller
 
     if ARGV.env
       args << "--env=#{ARGV.env}"
-    elsif formula.env.std? || formula.recursive_dependencies.any? { |d| d.name == "scons" }
+    elsif formula.env.std? || formula.deps.select(&:build?).any? { |d| d.name == "scons" }
       args << "--env=std"
     end