selftests/nolibc: always keep test kernel configuration up to date

Avoid using a stale test kernel configuration by always synchronizing
it to the current source tree.
kbuild is smart enough to avoid spurious rebuilds.

Shuffle the code around a bit to keep all the commands with side-effects
together.

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250123-nolibc-config-v2-5-5701c35995d6@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Author: t-8ch

tools/testing/selftests/nolibc/run-tests.sh

@@ -157,10 +157,6 @@ test_arch() {
 	fi
 	MAKE=(make -j"${nproc}" XARCH="${arch}" CROSS_COMPILE="${cross_compile}" LLVM="${llvm}" O="${build_dir}")
 
-	mkdir -p "$build_dir"
-	if [ "$test_mode" = "system" ] && [ ! -f "${build_dir}/.config" ]; then
-		swallow_output "${MAKE[@]}" defconfig
-	fi
 	case "$test_mode" in
 		'system')
 			test_target=run
@@ -173,6 +169,9 @@ test_arch() {
 			exit 1
 	esac
 	printf '%-15s' "$arch:"
+
+	mkdir -p "$build_dir"
+	swallow_output "${MAKE[@]}" defconfig
 	swallow_output "${MAKE[@]}" CFLAGS_EXTRA="$CFLAGS_EXTRA" "$test_target" V=1
 	cp run.out run.out."${arch}"
 	"${MAKE[@]}" report | grep passed