Force the inclusion of /usr/local/bin in PATH.

Sudo compiled with the --with-secure-path option enabled (default on
Ubuntu, Fedora and potentially other distros) will cause sudo to ignore
any changes made to the PATH variable in the user's environment (even when
the -E option is used). Also, some distros might not have /usr/local/bin
set in their sudoers secure_path causing the diskimage-builder to fail
when building an image using such distros as host. This patch will force
the inclusion of the /usr/local/bin in the PATH enviroment variable
inside the chroot.

Change-Id: Idb7156d8ff124bfe685cc721337d9bb21f49dc9e
Fixes: bug #1175980