appimagekit: set TOOLS_PREFIX to fix cross compilation

pkgs/tools/package-management/appimagekit/default.nix

···
     "-DUSE_SYSTEM_LIBARCHIVE=ON"
     "-DUSE_SYSTEM_GTEST=ON"
     "-DUSE_SYSTEM_MKSQUASHFS=ON"
+    "-DTOOLS_PREFIX=${stdenv.cc.targetPrefix}"
   ];
 
   postInstall = ''