build: fixup GHA deprecation warnings

[shoenig]

https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Unfortunately, actions/setup-go@v3 neither fixes all of the deprecation, and it breaks $GOROOT behavior we depend on.

We need https://github.com/actions/setup-go/issues/276 and https://github.com/actions/setup-go/issues/278 to be fixed, then we need a new release of https://github.com/magnetikonline/action-golang-cache pointed at setup-go@v4.