Always respect build options when invoking post_install

Fixes Homebrew/homebrew#36335.

Library/Homebrew/cmd/postinstall.rb

@@ -1,5 +1,5 @@
 module Homebrew
   def postinstall
-    ARGV.formulae.each {|f| f.post_install }
+    ARGV.formulae.each { |f| f.run_post_install }
   end
 end

Library/Homebrew/formula.rb

@@ -347,6 +347,12 @@ class Formula
     method(:post_install).owner == self.class
   end
 
+  # @private
+  def run_post_install
+    self.build = Tab.for_formula(self)
+    post_install
+  end
+
   # tell the user about any caveats regarding this package, return a string
   def caveats; nil end
 

Library/Homebrew/formula_installer.rb

@@ -594,7 +594,7 @@ class FormulaInstaller
   end
 
   def post_install
-    formula.post_install
+    formula.run_post_install
   rescue Exception => e
     opoo "The post-install step did not complete successfully"
     puts "You can try again using `brew postinstall #{formula.name}`"