chore: update workflows to use go.mod toolchain

[academician]

I noticed that PR #190 failed to build in CI because of some lints that have been introduced in go. This PR updates the github workflows to use the latest version of actions/setup-go which supports using a go-version-file (ie, go.mod which has a toolchain line). Running the workflow locally succeeded for me now.

Long-term it's probably a good idea to update the go version regularly. But this will unblock other PRs.