wip/aerc go modules downloading at build-time

[Kevin Bloom <kevin.bloom%posteo.net@localhost>]

Hi all,

I'm working the recipe for wip/aerc and I've run into an issue and
am unsure how to solve it. This program uses a GNU makefile but also
uses go. I've successfully created a go-modules.mk but no matter what
I try, it _always_ tried to download the modules again at build time.

I've even tried circumventing gmake with a custom go-build:
```
do-build:
       cd ${WRKSRC} && ${PKGSRC_SETENV} ${MAKE_ENV}
           GOPATH=${WRKDIR}/.gopath ${GO} build
```
This _still_ attempts to download the modules.

Any ideas?

Thanks,

Kev