bug#43075: Prioritize providing substitutes for security-critical packages with potentially long build times

[chaosmonk]

ungoogled-chromium receives frequent security updates, so it is 
important for users to keep it up-to-date.  However, binary substitutes 
for the latest version are usually not available, and it can take a  
very long time to build from source, possibly multiple days on low-end 
hardware.  This might tempt or force some users to put off upgrading 
the package and run an older, vulnerable version until a binary 
substitute is available or they have a chance to set aside the uptime 
needed to build from source.

I don't know what Guix's CI system looks like or how packages are 
queued for building, but if there is a way to prioritize builds for 
certain packages, I propose that substitutes for packages like 
ungoogled-chromium should be built as soon as possible once there is a 
new version.  Other security-critical packages with potentially long 
build times that come to mind are icecat and linux-libre.