cmd/compile/internal/importer,go/internal/gcimporter: use the 'go' command from build...

cmd/compile/internal/importer,go/internal/gcimporter: use the 'go' command from build.Default.GOROOT in lookupGorootExport

Also set GOROOT explicitly in case it is set to something else in the
caller's environment.

Fixes #59598.

Change-Id: I5599ed1183b23187fc3b976786f3c320d42ef4f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/484756
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>