goenv was making things more complicated than needed since it's really
designed to help with interactive use in a shell more than automated use
like this.

Instead, we'll follow the same strategy that our build.yml was doing of
just reading the .go-version file directly and then using the official
actions/setup-go action to do the actual installation. The key advantage
here is that Go ends up installed in a way where just running "go" will
do the right thing, and we no longer need to fuss with shims and
version-based path prefixes.

Rather than duplicating the logic from build.yml, instead it's factored
out into a separate composite action which both build.yml and checks.yml
will now share, in case we want to change the Go version selection
methodology in the future.