cmd/go: avoid recording GOROOT_FINAL in precompiled C archives

C archives for packages in GOROOT are shipped along with binary
releases of the Go toolchain. Although we build the toolchain with
GOROOT_FINAL set, we don't know actually know where the release will
be installed: the user's real GOROOT can differ arbitrarily from our
GOROOT_FINAL.

(In the specific case of toolchains installed through golang.org/dl
wrappers, the release's GOROOT_FINAL is /usr/local/go but the actual
GOROOT to which the release is installed is
$HOME/sdk/$(go env GOVERSION).)

Fixes #50183
Updates #48319

Change-Id: If10a42f90c725300bbcb89c3b5b01a2d93ab6ef7
Reviewed-on: https://go-review.googlesource.com/c/go/+/380915


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>