Let devs install microsoft/go similarly to `go install golang.org/dl/go1.16.6`

[dagood]

See https://golang.org/doc/manage-install#installing-multiple. (A partner team pointed it out to us as a way they get Go. In particular, to compare how different Go versions go when something strange is going on.)

https://github.com/golang/dl is a mirror of the golang.org/dl module's repo. It has a package for each version of Go: a small wrapper that can download that specific version to e.g. ~/sdk/go1.16.6, and after that, it redirects go1.16.6 [...] commands to that Go binary it downloaded.

The simplest-seeming thing to do would be to copy dl and provide a https://github.com/microsoft/go-dl module with e.g. a microsoft-go1.16.6 package that points at our blob storage instead of the tar.gz/zip files available on dl.google.com/go/.

[dagood]

It occurred to me that if we publish prebuilt copies of these commands, they could be considered "installers", and because they're written in Go, we would be more familiar with how to maintain them than if we create MSI (#29) or macOS pkg (#1325), RPM (#1318), or Deb (#9) installers.

Publishing a prebuilt binary fixes the bootstrap issue: to go install, you need go, but you don't need Go to run a prebuilt Go app.

For context: upstream doesn't publish prebuilt copies of golang.org/dl. But they already support MSI and macOS pkg installers.

[dagood]

A prototype version of this has been implemented internally (https://github.com/microsoft/go-lab/pull/142, thanks!) but it isn't done. A minimum Done for this issue IMO means:

• Available publicly.
• Automatically updates to include the latest version.

(Note that this is a public issue. We might want to create multiple internal issues to track parts of it if necessary.)