formula_installer: use formula from Keg for local bottle installs

Use the formula file from the Keg if `local_bottle_install` is set,
which means that the formula file from the tap is used for `-s`,
`--HEAD` and normal installs but the formula file present within
formula.prefix/".brew" is used for local bottle installs.

Library/Homebrew/formula_installer.rb

@@ -1074,9 +1074,14 @@ class FormulaInstaller
       -I #{$LOAD_PATH.join(File::PATH_SEPARATOR)}
       --
       #{HOMEBREW_LIBRARY_PATH}/postinstall.rb
-      #{formula.path}
     ]
 
+    args << if formula.local_bottle_path.present?
+      formula.prefix/".brew/#{formula.name}.rb"
+    else
+      formula.path
+    end
+
     Utils.safe_fork do
       if Sandbox.available?
         sandbox = Sandbox.new