Add "latest" tag to microsoft/go-infra-images, use it in microsoft/go

.NET is changing https://github.com/dotnet/dotnet-buildtools-prereqs-docker to produce latest tags that CI in each repo uses to keep up to date automatically with security fixes/rebuilds. Using a latest tag means each repo's CI will stay up to date without having constant PRs to update the CI files on each branch.

This might mean that a container change will break our CI sometimes, but it seems the .NET folks consider this a necessary risk. I think it's already mitigated decently: the CI logs do contain info about exactly which image is being used, so at the very least, we can observe that a change happened. In the worst case, we could change the microsoft/go repo to pin to an old image temporarily. (If, say, we are unable to produce new working images anymore.)

• https://github.com/dotnet/dnceng/issues/1313

Start by reviewing the microsoft/go-infra-images repository and the microsoft/go CI configuration, then read the linked dotnet/dnceng issue 1313 for the expected image-tagging workflow. Done means the infrastructure images publish latest tags, microsoft/go CI uses them, and the CI logs still identify the image being used.