only run post_install if it's defined

Closes Homebrew/homebrew#42593.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/postinstall.rb

@@ -2,7 +2,7 @@ require "sandbox"
 
 module Homebrew
   def postinstall
-    ARGV.resolved_formulae.each { |f| run_post_install(f) }
+    ARGV.resolved_formulae.select(&:post_install_defined?).each { |f| run_post_install(f) }
   end
 
   def run_post_install(formula)

Library/Homebrew/formula_installer.rb

@@ -400,11 +400,13 @@ class FormulaInstaller
     link(keg)
     fix_install_names(keg)
 
-    if build_bottle? && formula.post_install_defined?
-      ohai "Not running post_install as we're building a bottle"
-      puts "You can run it manually using `brew postinstall #{formula.full_name}`"
-    else
-      post_install
+    if formula.post_install_defined?
+      if build_bottle?
+        ohai "Not running post_install as we're building a bottle"
+        puts "You can run it manually using `brew postinstall #{formula.full_name}`"
+      else
+        post_install
+      end
     end
 
     ohai "Summary" if verbose? || show_summary_heading?