cmd/dist: log OS version when testing

As a follow-up to https://golang.org/cl/371474, add the OS version to
the metadata printed for each test.

This is a redo of CL 371475. This version updates go.mod and conforms to
the changes made in the parent commit.

Fixes #50146.

Change-Id: Iba5541cc8dd2c85c1fa3a215e30c8c3f9b6aaaab
Reviewed-on: https://go-review.googlesource.com/c/go/+/378590


Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>