zephyr-build-guide: define ZEPHYR_WORKSPACE before use

ZEPHYR_WORKSPACE is not defined before use. Defining it first, as
done in 'Build toolchains and SOF from sources' where SOF_WORKSPACE
is defined before use.

Signed-off-by: Suraj Sonawane <[email protected]>

Author: SurajSonawane2415

getting_started/build-guide/build-with-zephyr.rst

@@ -31,6 +31,7 @@ Initialize the west manifest ``$ZEPHYR_WORKSPACE/sof/west.yml`` using the ``west
 
       .. code-block:: bash
 
+         ZEPHYR_WORKSPACE=~/zephyr_workspace
          mkdir $ZEPHYR_WORKSPACE && cd $ZEPHYR_WORKSPACE
          west init -m https://github.com/thesofproject/sof