[bug#45006] cuirass: Add remote build support.

[Mathieu Othacehe <othacehe@gnu.org>]

Hey zimoun,

> Where are these specification_priority and package_priority configured?

specification_priority comes from the new "#:priority" field in the
Cuirass specification file and package_priority comes from the new
"#:priority" field in the job structure.

specification_priority ∈ [0, 9]
package_priority ∈ [0, 9]

 ⇒ build_priority ∈ [0, 99]

where 0 is the maximal priority. When both specification_priority and
package_priority are unset, the priority defaults to 99.

I'm currently using the following priorities:

modular: 1
guix-master:2
staging:3
core-updates:4

The builds are picked according to their priority and then their
timestamp, so that the most recent builds are picked first when the
priorities are identical.

I don't have a strategy regarding package_priority yet.

> This removes part of issues about GC on The Big Store, right?

Yes, this should allow to garbage collect way more aggressively the
store on berlin and hopefully reduce the GC duration.

Thanks,

Mathieu