update.sh: Use nix build from NixOS/nix#12107 to work around regressi…

…on of NixOS/nix#9708

I got tired of manually running

```
/nix/store/bn0djaymav6qa4zg5vaskzcaija5nq2p-nix-2.26.0pre20241227_9111645/bin/nix flake update
```

for every update while waiting for the fix to make it upstreaam.

update.sh

@@ -28,7 +28,8 @@ function do_update () {
 git submodule update --remote
 
 # Update flake.lock to match
-nix flake update
+# Use nix build from https://github.com/NixOS/nix/pull/12107 to work around regression of https://github.com/NixOS/nix/issues/9708
+/nix/store/bn0djaymav6qa4zg5vaskzcaija5nq2p-nix-2.26.0pre20241227_9111645/bin/nix flake update
 
 # Test that everything still builds
 tests/machine-builds.sh