Improve build and test experience for developers using windows about v7

[mkmik]

Circleci offers an experimental feature which enables builds coming from external forks.

However, it's an insecure option because it exposes all our project's conf, including environment variables containing API keys. We currently only use the coveralls API (actually we do it only in fossa at the moment, but we should do it on v7 as well), so it's not a big deal, but still...

It's not strictly necessary though, given that we could also just improve the build and experience for windows developers, which will also have a positive side effect of including the ecma report file in the PR.

[cpq]

Docker way sgtm

[mkmik]

• MSVC build with docker is already implemented.
• mingw cross target inside docker vm
• native mingw target