Use NPX to run prettier (#990)

### What

If `prettier` isn't installed globally, this means the format script
still works.

### How

If it's already installed, this is a no-op. Otherwise, this prompts the
user to download a temporary version of the package to run the command.

V3_GIT_ORIGIN_REV_ID: 6d10387a0a6d5cdc33b748c0533b7bb276d322ac

v3/flake.nix

@@ -217,7 +217,7 @@
                 pkgs.just
                 pkgs.moreutils
                 pkgs.nixpkgs-fmt
-                pkgs.nodePackages.prettier
+                pkgs.nodejs_22
 
                 # Rust
                 pkgs.bacon

v3/justfile

@@ -10,7 +10,7 @@ build:
 
 format:
   cargo fmt --check
-  prettier --check .
+  npx --yes prettier --check .
   ! command -v nix || nix fmt -- --check .
 alias fmt := format
 
@@ -21,7 +21,7 @@ fix:
   ! command -v nix || nix fmt
 
 fix-format:
-  prettier --write .
+  npx --yes prettier --write .
 
 run-local-with-shell:
   #!/usr/bin/env bash