trivial: workaround for ERROR: Unknown variable "gcab"
trivial: workaround for ERROR: Unknown variable "gcab"

This change enables the use of tests in Chrome OS. Fixes #3270

fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd