src: remove redundant v8 namespaces in `env.cc`

PR-URL: https://github.com/nodejs/node/pull/38792


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>