use VERCEL_ENV variable rather than a custom one

Author: Blargian

docusaurus.config.en.js

@@ -200,7 +200,7 @@ const config = {
         theme: {
           customCss: [require.resolve("./src/css/custom.scss")],
         },
-        ...(process.env.VERCEL_PREVIEW !== '1' && {
+        ...(process.env.VERCEL_ENV !== 'preview' && {
           googleTagManager: {
             containerId: 'GTM-WTNTDT7W',
           },