Change global.json rollForward to latestPatch once GitHub Actions supports it

[MattKotsenas]

Tracking issue for #170.

Once GitHub Actions supports the required rollForward strategies (tracked at https://github.com/actions/setup-dotnet/issues/448), we should switch from our current disable to latestPatch.

Alternatively, we can use renovate to auto-update global.json, but that only addresses CI and not local development.