Consider pre-installing clang / clang-runtime on plugin builders

[alanking]

While testing changes to pam_interactive, I find myself waiting a very long time for builds using the plugin builder even though compilation usually only takes a couple of seconds. I believe most of this time is spent downloading and installing the clang and clang-runtime externals (mostly clang).

It may be beneficial to offer at least the option to build the plugin builders with a version or versions of clang installed as part of the image rather than having to do it every time the build hook is run.