cmd/dist: add old style build tags

cmd/dist: add old style build tags

If one (accidentally) uses Go 1.16 as bootstrap toolchain, we
want it to print the error

found packages main (build.go) and building_Go_requires_Go_1_17_or_later (notgo117.go)

But because some files lack old style build tags, Go 1.16 instead
prints

//go:build comment without // +build comment

Add the build tags to make the error message work.

Change-Id: Iaa9b3c12e71842bb40f8687b2fda2cc4cb15b113
Reviewed-on: https://go-review.googlesource.com/c/go/+/431057
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>