cmd/go: change toolchain based on $GOTOOLCHAIN

cmd/go: change toolchain based on $GOTOOLCHAIN

For proposal #57001, add code to reinvoke a different Go toolchain
based on $GOTOOLCHAIN. The toolchain is searched for in $PATH
first and otherwise downloaded. The download is a standard module
download, so the toolchain is validated using the checksum database
before being executed or even stored in the file system.

Followup CLs will refine the exact toolchain selection and implement
other parts of the proposal. This is only the download+reinvoke code.

For #57001.

Change-Id: I44363cbd916dac01342b1bfce6a487fe7166be4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/475955
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>