If we're using -mod=vendor then we effectively load
a fake build list from vendor/modules.txt.
Do not write it back to go.mod.

Fixes #26704.

Change-Id: Ie79f2103dc16d0b7fe0c884e77ba726c7e04f2e4
Reviewed-on: https://go-review.googlesource.com/128899Reviewed-by: Bryan C. Mills <bcmills@google.com>