bug#73916: [Cuirass] Queued derivations have their GC roots removed

[Ludovic Courtès]

Cuirass stores GC roots for all the derivations of its jobsets under
/var/guix/profiles/per-user/cuirass/cuirass (or similar).

The ‘delete-old-gc-roots’ periodically deletes these GC roots (which are
not just derivations), but it’s supposed to keep GC roots to derivations
that are queued: that was done in
55af0f70c0d4938b8eda777382bbc4d8f5698a37.

Unfortunately, there’s ample evidence on berlin that GC roots of .drv
currently queued get removed anyway.  This causes workers to try in vain
to substitute them, to eventually reject the build, which gets canceled
by ‘remote-server’.

Ludo’.