build: use the system's ccache by default

* ccache gets updated semi-frequently and some builders would like
  to be able to take advantage of new features without having to
  rely on the prebuilt version getting updated

* If a build system has ccache installed already, use that version
  instead of the prebuilt

Change-Id: I8988c8a25fab3694d84633f957c2b92ce84cf69e