zephyr-env: don't fail unnecesarily when 'set -e'

The way the PATH modification bit was set was failing unnecesarily
when the shell is being ran with -e (fail on error), which is leading
to unnecessary (and dangerous) workarounds in integration scripts.

So wrap it in an if sentence, so the shell doesn't bail out when the
grep fails to find the scripts_path in PATH.

Change-Id: Ia88a5f430e08ef4c186d11834f4cd840ccd29f7f
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>