nixos-install: more robust way of sourcing fresh version of self

nixos/modules/installer/tools/nixos-rebuild.sh

@@ -214,9 +214,9 @@ fi
 
 # Re-execute nixos-rebuild from the Nixpkgs tree.
 if [ -z "$_NIXOS_REBUILD_REEXEC" -a -n "$canRun" ]; then
-    if p=$(nix-instantiate --find-file nixpkgs/nixos/modules/installer/tools/nixos-rebuild.sh "${extraBuildFlags[@]}"); then
+    if p=$(nix-build --no-out-link --expr 'with import <nixpkgs/nixos> {}; config.system.build.nixos-rebuild' "${extraBuildFlags[@]}"); then
         export _NIXOS_REBUILD_REEXEC=1
-        exec $SHELL -e $p "${origArgs[@]}"
+        exec $p/bin/nixos-rebuild "${origArgs[@]}"
         exit 1
     fi
 fi