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.

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

To actually fix the coverage on https://coverage.nodejs.org/ I'll also need to make some changes (https://github.com/nodejs/build/pull/2705) on the build infra side to the machine that the node-test-commit-linux-coverage-daily job runs on but that shouldn't be needed if you're running coverage yourself on a more recent OS/Linux distribution (the complication on the CI host stems from running Ubuntu 16.04).