Fix Linux AppImage bundling of deps-built shared libraries

The AppImage dependency closure resolves each bundled ELF's DT_NEEDED
entries with plain ldd, which cannot resolve the deps-built FFmpeg stack
(libavcodec/libavutil/libswscale) once it is copied into the bundle:
those libs are not installed in any standard loader path and carry no
RUNPATH of their own, so ldd reports the siblings as missing and the
build aborts. Extend the loader path with the bundle directory plus the
source directories of already-bundled files (mirroring
scripts/check_appimage_libs.sh), and key the dedup set on the bundled
file path instead of the source path so dependencies resolved from the
bundle directory are not copied onto themselves.

Co-Authored-By: Claude <noreply@anthropic.com>

src/dev-utils/platform/unix/build_linux_image.sh.in

@@ -83,6 +83,10 @@ copy_shared_object_to_dir() {
     src_real="$(readlink -f "$src")"
     dst_name="$(basename "$src_real")"
     mkdir -p "$dst_dir"
+    if [ "$src_real" = "$dst_dir/$dst_name" ]; then
+        # Already bundled; the dependency resolved from the bundle directory.
+        return 0
+    fi
     cp -fL "$src_real" "$dst_dir/$dst_name"
 
     if [ -L "$src" ]; then
@@ -101,7 +105,7 @@ bundle_dependency_closure() {
     shift
 
     local -a queue=("$@")
-    local target dep dep_real copied_path
+    local target dep dep_real dep_key copied_path
     declare -A seen=()
     # Dependencies are resolved with ldd, which only searches the default
     # loader path. Deps-built shared libraries (e.g. the FFmpeg stack) are not
@@ -133,11 +137,17 @@ bundle_dependency_closure() {
                 continue
             fi
 
-            if [ -n "${seen[$dep_real]}" ]; then
+            # Key dedup on the bundled file rather than the source path: once
+            # ldd resolves a library from the bundle directory (via the
+            # LD_LIBRARY_PATH above) its path is a dst_dir path, which differs
+            # from the source path the first resolution returned. Keying on
+            # the source path would re-copy the file onto itself.
+            dep_key="$dst_dir/$(basename "$dep_real")"
+            if [ -n "${seen[$dep_key]}" ]; then
                 continue
             fi
 
-            seen[$dep_real]=1
+            seen[$dep_key]=1
             copy_shared_object_to_dir "$dep" "$dst_dir"
             search_dirs+=("$(dirname "$dep_real")")
             copied_path="$dst_dir/$(basename "$dep_real")"