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).)