cmd/link: fix defaultGOROOT package
CL 310731 moved cmd/internal/objabi.defaultGOROOT to
internal/buildcfg.defaultGOROOT, but didn't update the place in the
linker that sets its value.
Fixes the failing reboot test on the GOEXPERIMENT builders.
Change-Id: I135b6bfc0fdadbe6cfc144d7aa55ca13519ba004
Reviewed-on: https://go-review.googlesource.com/c/go/+/310869
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>