Use HOMEBREW_LOGS more consistently.

Library/Homebrew/cmd/doctor.rb

@@ -381,8 +381,7 @@ end
 end
 
 def check_access_logs
-  folder = Pathname.new('~/Library/Logs/Homebrew')
-  if folder.exist? and not folder.writable_real?
+  if HOMEBREW_LOGS.exist? and not HOMEBREW_LOGS.writable_real?
     <<-EOS.undent
       #{folder} isn't writable.
       This can happen if you "sudo make install" software that isn't managed

Library/Homebrew/exceptions.rb

@@ -200,7 +200,7 @@ class BuildError < Homebrew::InstallationError
       Homebrew.dump_build_env(env)
       puts
       onoe "#{formula.name} did not build"
-      unless (logs = Dir["#{ENV['HOME']}/Library/Logs/Homebrew/#{formula}/*"]).empty?
+      unless (logs = Dir["#{HOMEBREW_LOGS}/#{formula}/*"]).empty?
         puts "Logs:"
         puts logs.map{|fn| "     #{fn}"}.join("\n")
       end