GitHub Actions: only run on master branch

[ethomson]

At present, GitHub Actions is configured to build on every pull request and push to any branch.

This means that CI runs will run in duplicate for pull requests that originate from branches in the repository (ie, those not from a fork). That is to say: if you push a branch foo and then open a pull request from foo to master, you'll see duplicate builds, one for the push into foo and one for the pull request into master.

By limiting this to pushes to master and pull_requests to master, this will remove the dual build problem.

/cc @joaomoreno

[ethomson]

Should I include other branches, like release branches here?

[ethomson]

Thanks @joaomoreno - that's exactly the syntax. I've pulled in those changes.