fix: use npm_config_user_agent, which is more reliable

Ref facebook/create-react-app#11322 (comment)

index.js

@@ -358,12 +358,8 @@ async function init() {
   // Supported package managers: pnpm > yarn > npm
   // Note: until <https://github.com/pnpm/pnpm/issues/3505> is resolved,
   // it is not possible to tell if the command is called by `pnpm init`.
-  const packageManagerBinary = path.basename(process.env.npm_execpath || '')
-  const packageManager = /pnpm/.test(packageManagerBinary)
-    ? 'pnpm'
-    : /yarn/.test(packageManagerBinary)
-    ? 'yarn'
-    : 'npm'
+  const userAgent = process.env.npm_config_user_agent ?? ''
+  const packageManager = /pnpm/.test(userAgent) ? 'pnpm' : /yarn/.test(userAgent) ? 'yarn' : 'npm'
 
   // README generation
   fs.writeFileSync(