Fix distcc when used with ccache

Using distcc when ccache is also set in makepkg.conf can result in unbuildable projects that rely on CMake. In this scenario, CMake will test for compilers and fail to build its simple test program due to distcc being recursively invoked.

According to distcc INSTALL, the correct way of using distcc with ccache is simply prepending distcc to PATH with higher priority than ccache, thus removing the CCACHE_PREFIX from the environment.

I have tested this fix with an AUR package, and the fix produced a buildable package, and both distcc and ccache were invoked.