Include `svt-av` in the AppImage rootfs for external tools

Initially posted as a question, see: https://github.com/ivan-hc/Steam-appimage/discussions/86. Tools such as `gamescope` cannot be run inside steam launch configuration commands due to missing shared libraries (eg, `libSvtAv1Enc.so.3`) which are provided by the `svt-av` package on archlinux (which the rootfs is based on).