formula_installer.rb: Improve receipt check

Using `f.installed?` instead of `f.prefix.exist?` ensures something was created
by the installation step before the install receipt is written.

Library/Homebrew/formula_installer.rb

@@ -131,7 +131,7 @@ class FormulaInstaller
       raise "Suspicious installation failure" unless $?.success?
 
       # Write an installation receipt (a Tab) to the prefix
-      Tab.for_install(f, args).write if f.prefix.exist?
+      Tab.for_install(f, args).write if f.installed?
     end
   end