test-bot: always set PATH to HOMEBREW_PREFIX.

Set the `PATH` to always have the `bin` and `sbin` for the current
`HOMEBREW_PREFIX` at the beginning.

Library/Homebrew/dev-cmd/test-bot.rb

@@ -1021,6 +1021,7 @@ module Homebrew
     ENV["HOMEBREW_NO_EMOJI"] = "1"
     ENV["HOMEBREW_FAIL_LOG_LINES"] = "150"
     ENV["HOMEBREW_EXPERIMENTAL_FILTER_FLAGS_ON_DEPS"] = "1"
+    ENV["PATH"] = "#{HOMEBREW_PREFIX}/bin:#{HOMEBREW_PREFIX}/sbin:#{ENV["PATH"]}"
 
     travis = !ENV["TRAVIS"].nil?
     if travis