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>

scripts/jobserver-exec

@@ -23,7 +23,9 @@ try:
 	opts = [x for x in flags.split(" ") if x.startswith("--jobserver")]
 
 	# Parse out R,W file descriptor numbers and set them nonblocking.
-	fds = opts[0].split("=", 1)[1]
+	# If the MAKEFLAGS variable contains multiple instances of the
+	# --jobserver-auth= option, the last one is relevant.
+	fds = opts[-1].split("=", 1)[1]
 	reader, writer = [int(x) for x in fds.split(",", 1)]
 	# Open a private copy of reader to avoid setting nonblocking
 	# on an unexpecting process with the same reader fd.