Fix: "$_" does not expand as expected on Ubuntu

Fix: "$_" does not expand as expected on Ubuntu

While using Ubuntu as default builder nodes.
The .bashrc file does not process '$_' expanding
the output correctly as provided in previous line.
Therefore explicitly use the dirname or use /bin/bash.

This results in a copy error.

This happens is because by default Ubuntu "/bin/sh"
points to "/bin/dash" while on CentOS / RedHat points to
"/bin/bash".

Issue: RELENG-5666
Change-Id: Ibb4e0daeaf9730464451e229289b687aa2fb5389
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>