libvmaf: replace xxd dependency with minimal shell shim

libvmaf uses `xxd --include` to embed model JSON files as C arrays.
This pulled in tinyxxd, causing ~22k rebuilds via libaom -> ffmpeg ->
everything whenever tinyxxd was updated. Replace it with a minimal
`xxd --include` implementation using only stdenv tools (od, sed).

The output is bit-for-bit identical (verified with diffoscope).

pkgs/by-name/li/libvmaf/package.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  buildPackages,
   stdenv,
   fetchFromGitHub,
   ffmpeg-full,
@@ -8,7 +9,6 @@
   nasm,
   ninja,
   testers,
-  xxd,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -28,7 +28,7 @@ stdenv.mkDerivation (finalAttrs: {
     meson
     ninja
     nasm
-    xxd
+    (buildPackages.callPackage ./xxd.nix { })
   ];
 
   postPatch = lib.optionalString stdenv.hostPlatform.isFreeBSD ''

pkgs/by-name/li/libvmaf/xxd.nix

@@ -0,0 +1,20 @@
+# Minimal `xxd --include` replacement for embedding binary files as C arrays.
+# Used instead of tinyxxd to avoid ~22k rebuilds via libaom -> ffmpeg -> everything.
+{ writeShellScriptBin }:
+
+writeShellScriptBin "xxd" ''
+  if [ "$1" != "--include" ]; then
+    echo "xxd: only --include mode is supported" >&2
+    exit 1
+  fi
+  input="$2"
+  output="$3"
+  # Match real xxd behavior: derive variable name from the full path
+  varname=$(echo "$input" | sed 's/[^a-zA-Z0-9_]/_/g')
+  {
+    printf 'unsigned char %s[] = {\n' "$varname"
+    od -An -tx1 -v "$input" | sed 's/  */ /g; s/^ //; s/ $//; s/ /, 0x/g; s/^/  0x/; s/$/,/'
+    printf '};\n'
+    printf 'unsigned int %s_len = %d;\n' "$varname" "$(wc -c < "$input")"
+  } > "$output"
+''