cmd/go/internal/modload: make it clear -mod can't be set in some cases
We check that -mod can't be set to mod in workspace mode, but then we
set BuildMod to mod for go work sync below. Make it clear that that's
okay because we can't pass -mod=mod to go work sync (or the other go
mod commands that can run in workspace mode that set mod=mod: go mod
graph, go mod verify, and go mod why).
Change-Id: Idfe6fea6a420211886e4f838e050be4bf7d1b71d
Reviewed-on: https://go-review.googlesource.com/c/go/+/497617 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>