dim: fix shellcheck warning in ask_user

Fix:

local prompt="$@ (y/N) "
               ^-- SC2124: Assigning an array to a string! Assign as
               array, or use * instead of @ to concatenate.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: b5403812 ("dim: ask the user to create DIM_PREFIX and clone DIM_REPO")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>