Add github action for CI of Linux

[bovine]

Continuous Integration support for Linux compilation using Github Actions.

[mutability]

What does this actually turn on? We already have automated package building running via the Jenkinsfile.

[mutability]

Seems like this is an x86 build on ubuntu? That's, I guess, useful to check for compatibility reasons, but it won't produce packages that we'd release.

[bovine]

Right, this isn't intended to produce final binaries for release purposes, just to ensure compilability of the codebase and submitted PRs before they are merged. By being run on the github side (rather than Jenkins), the public can see any build failures of their own code before submitting PRs to us, even if they fork this repo into their own user account.