Temporarily adding -vgc to a DUB build

[Nordlöw via Digitalmars-d-learn]

How do I temporarily enable -vgc when building my app with DUB?
I've tried

DFLAGS=-vgc /usr/bin/dub build --build=unittest

but it doesn't seem to have any effect as it doesn't rebuild 
directly after the call

/usr/bin/dub build --build=unittest

I'm using DUB version 1.5.0

Or is adding a new build configuration, say unittest-vgc, the 
only way to accomplish this?