Run golangci-lint in CI

See https://golangci-lint.run

This has already helped uncover some issues, and we should make more use
of it. Currently we have to disable one of the default lints, as we have
a lot of issues it complains about. I plan to address those one by one
and enable it. Until then, put this in place to make sure we don't
re-introduce any of the issues we've eliminated so far.