gitlab-ci: Prepend to PATH rather than replace it

Currently we set the entire PATH rather than prepend the new paths that
we need to have searched.  This however breaks parts of the "virtualenv"
that was have set up and need to use as that also will be modifying
PATH.  To fix this, prepend our new locations instead.

Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Rini <trini@konsulko.com>