build: update gcovr for gcc 8 compatibility

Update the version of `gcovr` used for C++ coverage from 3.4 to 4.2 for
compatibility with gcc/g++ 8.

PR-URL: https://github.com/nodejs/node/pull/39326
Refs: https://github.com/nodejs/node/issues/39303
Refs: https://github.com/gcovr/gcovr/pull/228
Refs: https://github.com/nodejs/build/pull/2705


Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>