cmd/go: add mod vendor -check

[fishy]

Very similar to #27005, I think a go mod vendor -check that returns non-zero when it would make any changes to vendor/ directory would be useful in pre-commit hooks and CIs.

Things that could be different from go mod tidy -check are:

1. Unlike go mod tidy -check, this likely requires network access to work
2. This might be more go version dependent? I think the vendor/modules.txt file are different between 1.15 and 1.14, not sure if this will cause problems with proposed go mod vendor -check.

[bcmills]

Duplicate of #27348?

[fishy]

Yea I think #27348 pretty much covered it 👍