cmd/go: include the "-trimpath" flag in the stamped build settings

cmd/go: include the "-trimpath" flag in the stamped build settings

The -trimpath flag has a strong effect on the resulting binary:
in particular, it determines whether runtime.GOROOT can report
a meaningful path in the absence of an explicit GOROOT environment variable.

For #51461

Change-Id: Id0d55572c0a0a4e2e4724363ed80dfa05b202186
Reviewed-on: https://go-review.googlesource.com/c/go/+/391810
Trust: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>