Fix: download composer directly instead of using Nix package

nixpacks.toml

@@ -12,12 +12,14 @@ nixPkgs = [
   "php84Extensions.fileinfo",
   "php84Extensions.openssl",
   "nodejs_22",
-  "nodePackages.npm",
-  "composer"
+  "nodePackages.npm"
 ]
 
 [phases.install]
 cmds = [
+  "php -r \"copy('https://getcomposer.org/installer', 'composer-setup.php');\"",
+  "php composer-setup.php --install-dir=/usr/local/bin --filename=composer",
+  "php -r \"unlink('composer-setup.php');\"",
   "composer install --no-dev --optimize-autoloader --no-interaction",
   "npm ci"
 ]