Node.js 12 TODO: Switch to Linux for generating source & headers tarballs

[rvagg]

This is currently done on macOS along with docs. Depending on the workflow we might want to switch docs to Linux too. This has been a persistent problem (e.g. #1658) and is to do with the difference between the macOS / BSD tar and GNU tar. I believe GNU tar provides us with maximum compatibility and should make errors go away.

For safety I think we should make this change only happen for Node >= 12. It's going to require some work in the VersionSelectorScript and/or ci-release/iojs+release scripts to switch logic.

[rvagg]

solved in #1777, nightlies as of today use centos7 to generate all non-binary assets.