New review comment by ericonr on void-packages repository

https://github.com/void-linux/void-packages/pull/28129#discussion_r567336350

Comment:
I like the idea, but I'd prefer to have it as a general helper, otherwise this will have to be repeated across a few INSTALL scripts (including XBPS; it's not a huge issue there because the gid for `xbuilder` is constant, but still not great).

The helper shouldn't be added here though, so I will do it afterwards.