raspberrypi-builder: ensure scripts fails on error

The way this ends up being called with the raspberry pi 4 image builder
ends up not using the `-e` from the shebang.

In turn, the builds fails during cross-compilation. The wrong coreutils
ends up being used, but this is not made apparent.

The issue I faced is already fixed on master, but this ensures no one
ends up with a failed build "succeeding".

nixos/modules/system/boot/loader/raspberrypi/raspberrypi-builder.sh

@@ -1,4 +1,7 @@
-#! @bash@/bin/sh -e
+#! @bash@/bin/sh
+
+# This can end up being called disregarding the shebang.
+set -e
 
 shopt -s nullglob