tools: fix v8 testing with devtoolset on ppcle

The devtoolset doesn't use or set the CXX, etc, env vars, so ignore them
if not present.

PR-URL: https://github.com/nodejs/node/pull/28458


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>