Cope with e.g. host /usr/lib being an absolute symlink to /usr/lib64

Low priority unless we find out that another distro does this. Gentoo used to do this, but such versions are officially unsupported now.

https://github.com/ValveSoftware/steam-runtime/issues/305

Ideally pressure-vessel would be able to deal with this. I think I fixed something similar in Flatpak (in commits 12e3dc05 and 08d65c54), but the code that pressure-vessel uses to set up /run/host is not identical.

I think we should ideally use flatpak_exports_append_bwrap_args() directly to set up /run/host - that way, what we do will be the same as Flatpak.