Fix up envvar exports for the apt buildpack -- fixes C/C++ compiling.

 * Avoid adding extra : characters to the end of gcc paths.  This will
 cause gcc to use the current directory in system include paths, which
 breaks C #include <file> vs #include "file" semantics.
 * Also, write an export file for buildpack-multi to pass the exported
 envvars to the next buildpack.