gs-install-appstream: add gs_private_libdir to rpath

This avoids a crash where the executable crashed due to not being able to load the library.

I have encountered the error below testing gnome-software 41.1 in alpine, but the problem is still present in main:

Error loading shared library libgnomesoftware.so.16: No such file or directory (needed by /usr/libexec/gnome-software-install-appstream)
Error relocating /usr/libexec/gnome-software-install-appstream: gs_external_appstream_utils_get_file_cache_path: symbol not found
GS Failed to update external appstream file: Child process exited with code 127

I assume this was introduced in !601 (merged) or at a later point by adding the gs_external function. I have not been able to find a related open issue. Did I miss it or has nobody encountered this before? In second case, maybe is this specific to alpine?