nixos-rebuild: get Nix from channel

If our old Nix can’t evaluate the Nixpkgs channel, try the fallback
from the new channel /first/. That way we can upgrade Nix to a newer
version and support breaking changes to Nix (like seen in the upgrade
o Nix 2.0).

This change should be backported to older NixOS versions!

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

@@ -260,6 +260,14 @@ if [ -n "$rollback" -o "$action" = dry-build ]; then
     buildNix=
 fi
 
+nixSystem() {
+    machine="$(uname -m)"
+    if [[ "$machine" =~ i.86 ]]; then
+        machine=i686
+    fi
+    echo $machine-linux
+}
+
 prebuiltNix() {
     machine="$1"
     if [ "$machine" = x86_64 ]; then
@@ -279,7 +287,9 @@ if [ -n "$buildNix" ]; then
     nixDrv=
     if ! nixDrv="$(nix-instantiate '<nixpkgs/nixos>' --add-root $tmpDir/nix.drv --indirect -A config.nix.package.out "${extraBuildFlags[@]}")"; then
         if ! nixDrv="$(nix-instantiate '<nixpkgs>' --add-root $tmpDir/nix.drv --indirect -A nix "${extraBuildFlags[@]}")"; then
-            nixStorePath="$(prebuiltNix "$(uname -m)")"
+            if ! nixStorePath="$(nix-instantiate --eval '<nixpkgs/nixos/modules/installer/tools/nix-fallback-paths.nix>' -A $(nixSystem) | sed -e 's/^"//' -e 's/"$//')"; then
+                nixStorePath="$(prebuiltNix "$(uname -m)")"
+            fi
             if ! nix-store -r $nixStorePath --add-root $tmpDir/nix --indirect \
                 --option extra-binary-caches https://cache.nixos.org/; then
                 echo "warning: don't know how to get latest Nix" >&2