build: make sure /bin, /usr/bin are in $PATH

eg. on archlinux /bin is a symlink to /usr/bin, and /bin is not
in $PATH.  building rpm packages for openSUSE fails because the
chroot environment has /bin/rpm invoked as `chroot $BUILD_ROOT rpm`.
Merge pull request #100 from roman-neuhauser/sanitize-PATH

build: make sure /bin, /usr/bin are in $PATH