steam: fix /etc/resolv.conf reference in FHS env

It seems as if it's a problem if `/etc/resolv.conf` is a symlink to
`/run/systemd/resolve/stub-resolv.conf` which is the case when using
`systemd-resolved.service`:

    bwrap: Can't bind mount /oldroot/etc/resolv.conf on /newroot/etc/resolv.conf: Unable to mount source on destination: No such file or directory

I confirmed that by following the symlink of `/etc/resolv.conf`
(pointing to `/run/systemd/resolve/stub-resolv.conf`) with `readlink -f`
the issues are all gone.

pkgs/build-support/build-fhs-userenv-bubblewrap/default.nix

@@ -70,7 +70,7 @@ let
       "pki"
     ];
   in concatStringsSep "\n  "
-  (map (file: "--ro-bind-try /etc/${file} /etc/${file}") files);
+  (map (file: "--ro-bind-try $(${coreutils}/bin/readlink -f /etc/${file}) /etc/${file}") files);
 
   # Create this on the fly instead of linking from /nix
   # The container might have to modify it and re-run ldconfig if there are