steam: switch to correct runtime tarball

In my previous commit (593c28b8) I used the wrong upstream artifact for
the runtime. After reading the documentation in the
ValveSoftware/steam-runtime repo, I now know that the steam-runtime
tarball is what I actually wanted.

I also used 'diff' to compare the various artifacts with the old runtime
this package used before, and the steam-runtime one is certainly the
closest.

Most importantly, switching to the right steam-runtime package
reportedly fixes issues for other users (fixes #90229).

This also entirely removes the amd64/i386 split from runtime.nix because
the upstream package bundles both together, and if that's how upstream
wants to distribute this, it seems best to follow their lead.