appimage: Fix RUNPATH for libexec

* docker/appimage/build-appimage.sh: Move to ...
* src/appimage/build-appimage.sh: here.  Fixup libexec.
* docker/run-appimage-build.sh: Adjust.
* docker/appimage/Dockerfile: Do not copy the build script.

This is a simple hack because linuxdeploy or any of its standard
plugin do known about libexec. We simply run patchelf directly.

Moving the build script makes development easier.