src: suppress warning in src/node_env_var.cc

Initialise the superclass object in the copy constructor of mapKVStore

PR-URL: https://github.com/nodejs/node/pull/31136
Refs: https://github.com/nodejs/node/issues/18983


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>