meson: don't use intermediate variables that are immediately discarded

meson: don't use intermediate variables that are immediately discarded

For things like:
loop
    x = func()
    list += x
end

just do:
loop
    list += func()
end

Acked-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>