Ungrafting CI jobs

[Leo Famulari]

We started building the wip-ungrafting branch a week ago:

https://ci.guix.gnu.org/jobset/ungrafting

Some progress has been made, but not enough. This is partially due to
<https://bugs.gnu.org/48574>, and also due to inefficient building of
expensive bootstrap paths (namely, every Rust package derivation tried
to build the entire Rust dependency graph concurrently).

I just rebased the branch and pushed again.