cmd/go: set GOROOT explicitly for 'go generate' subprocesses

cmd/go: set GOROOT explicitly for 'go generate' subprocesses

Code generators may reasonably expect to find the GOROOT for which the
code is being generated.

If the generator invokes 'go run' (which ought to be reasonable to do)
and the user has set 'GOFLAGS=trimpath' (which also ought to be
reasonable), then either 'go generate' or 'go run' needs to set GOROOT
explicitly.

I would argue that it is more appropriate for 'go generate' to set
GOROOT than for 'go run' to do so, since a user may reasonably invoke
'go run' to reproduce a user-reported bug in a standalone Go program,
but should not invoke 'go generate' except to regenerate code for a Go
package.

Updates #51461.

Change-Id: Iceba233b4eebd57c40cf5dcd4af9031d210dc9d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/399157
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>