Make zephyr-env.sh work when cd is aliased

Force the zephyr-env.sh script to invoke the builtin 'cd' command so
that the script correctly assigns ZEPHYR_BASE, even for users who
alias the 'cd' command.

Aliasing 'cd' to emit a string before changing the directory is one
way of putting the current working directory in the title of a
terminal window. If the user's alias is executed in zephyr-env.sh, the
output of 'cd' pollutes ZEPHYR_BASE, resulting in value which won't
build.

Signed-off-by: Matt Braun <matthew.braun@intel.com>
Change-Id: I8d7810d1a44ec14102d07bf35610b47d06ac1015