arch packaging: exclude assets from PKGBUILD rsync

Exclude the assets/ directory from the PKGBUILD rsync step to avoid
permission issues (e.g. map.png) when building the Arch package in
Docker as aur_builder.

https://chatgpt.com/share/693a8c25-4464-800f-8d5e-5c4579d78b52

packaging/arch/PKGBUILD

@@ -34,6 +34,7 @@ prepare() {
     --exclude="pkg" \
     --exclude="srcpkg" \
     --exclude="packaging" \
+    --exclude="assets" \
     "$project_root/" "$srcdir/$_srcdir_name/"
 }