cmd/go: enter vendor mode depending on new modules.txt workspace line

cmd/go: enter vendor mode depending on new modules.txt workspace line

modules.txt gets a new ## workspace line at the start of the file if
it's generated in workspace mode. Then, when deciding whether the go
command runs in mod=vendor, we only do so if we're in the same mode
(workspace or not) as the modules.txt specifies.

For #60056

Change-Id: If478a9891a7135614326fcb80c4c33a431e4e531
Reviewed-on: https://go-review.googlesource.com/c/go/+/513756
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>