scripts/jobserver-exec: parse the last --jobserver-auth= option

In the GNU Make manual, the section "Sharing Job Slots with GNU make"
says:

    Be aware that the MAKEFLAGS variable may contain multiple instances
    of the --jobserver-auth= option. Only the last instance is relevant.

Take the last element of the array, not the first.

Link: https://www.gnu.org/software/make/manual/html_node/Job-Slots.html


Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>