cmd/internal/testdir: re-enable default all codegen flag on linux-amd64
This was limited to just the mainline linux-amd64 builder, but we don't
use that name anymore. Use the LUCI name instead, gotip-linux-amd64.
Change-Id: Ib4377ad336c529512d9939ff9dce0ea242528b74
Reviewed-on: https://go-review.googlesource.com/c/go/+/703136
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>