bug#33124: Cuirass: register GC roots for evaluations under /var/guix/gcroots

["Clément Lassieur" <clement@lassieur.org>]

I think the goal here is to make sure that the builds that resulted from
of 1) important evaluations (releases) and 2) recent evaluations aren't
garbage collected.

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<sneek> snape, civodul says: i think Cuirass should register GC roots for its
	evaluations under /var/guix/gcroots
<snape> [...] How many such evaluations should be registered?  If all
	are registered, we'll run out of space won't we?  So would it be
	like the last 15 or something?
<civodul> snape: i think we'd register all of them and use (guix cache) to
	  automatically wipe old ones periodically  [09:53]
<civodul> snape: when there's a release we could manually stash important
	  evaluations away so they won't be deleted
<civodul> so i'd just create one subdir for each eval say in
	  /var/guix/profiles/USER/cuirass/evaluations  [09:54]
<civodul> and that'd contain a bunch of symlinks to both the .drv and the
	  outputs
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Clément