Place cmd folders in taps at front of brew's PATH

Per conversation on [GitHub][1], it makes sense for brew's internal PATH to put
cmd folders in taps at the front rather than the end. This way, people who
update external commands that switch to this style of external command will get
the benefits, even if they forget to uninstall the older version.

[1]: https://github.com/telemachus/homebrew-desc/issues/54
Let's interpolate it instead e.g. `ENV["PATH"] = "#{tap_cmd_dir}#{File::PATH_SEPARATOR}#{ENV["PATH"]}"