goa build fail with `-mod=vendor`

[wyxloading]

Describe the bug
goa build ... fail, error log like this

goa build -mod=vendor -o build/demo main.go plugin.go
-mod=vendor -o /Users/wyx/demo/build/demo main.go plugin.go not supported (can be '', 'readonly', or 'vendor')

To Reproduce
Steps to reproduce the behavior:

1. goa build with more options, like -mod=vendor, -race

Expected behavior
Should build without any problem

Screenshots
None

Desktop (please complete the following information):

• OS: Mac OS 10.14.6 (18G103)
• go version 1.13