release: force PKGCONFBRANCH to use 'latest' to fix BETA1 builds

We do not yet have a 'quarterly' build for amd64, i386, or non-x86,
so use 'latest' in order to get the BETA1 builds to work.

Approved by: re (implicit, build fix)
Sponsored by: GoFundMe https://www.gofundme.com/f/gjbbsd
Sponsored by: PayPal https://paypal.me/gjbbsd