Move FlexBridge NuGet packaging build to GitHub Actions

[rmunn]

The build that creates the NuGet package(s) for FlexBridge is currently running on TeamCity. It's been failing to build since the commit that updated SIL.Chorus.Mercurial to 6.5.1. The reason, at least for the Linux64 build, is because the TeamCity runner is still on Ubuntu Xenial, which has Python 3.5. Mercurial, starting with release 6.2, supports Python 3.6 (or above) only.

We could fix this by creating a new TeamCity Linux build agent, but we're trying to move away from TeamCity. The better solution would be to move the NuGet packaging build to GitHub Actions.