Fix AppImage build and launch

Use an older version of linuxdeploy to work around a FUSE memory
allocation error (see
https://discuss.pixls.us/t/countdown-to-5-12/49160/6).

Override the minimum CMake version when building libjxl to fix the CMake
configuration error for the sjpeg dependency.

tools/makedeb/PKGBUILD.libjxl

@@ -25,6 +25,7 @@ build() {
   local _cmake_options=(
     -B build
     -S "$_pkgsrc"
+    -DCMAKE_POLICY_VERSION_MINIMUM=3.5
     -DCMAKE_BUILD_TYPE=Release
     -DCMAKE_INSTALL_PREFIX='/usr'
     -DJPEGXL_FORCE_SYSTEM_BROTLI=ON