bug#42371: Grafts max out build slots without being queued

[Maxim Cournoyer]

Hello everyone,

I did a couple more tests, and there's no need to get fancy with
the reproduction steps; any number greater of packages greater than your
number of build slots/users should suffice:

--8<---------------cut here---------------start------------->8---
guix build --quiet --check $(guix package -A | head -n 11 | cut -f1)
The following derivations will be built:
   /gnu/store/l7hrbi8cikf4c0calvqwd2n99v51ljzb-aalib-1.4rc5.drv
   /gnu/store/bijqg7w67rq1lr20hqbk18rq73963xfr-a2ps-4.14.drv
   /gnu/store/vh283nmxk08sl0m89s1fn07m432c9dg0-4ti2-1.6.9.drv
   /gnu/store/davmjim4nfvbhimpim329p642p9mg50d-0xffff-0.8.drv
   /gnu/store/7qaf91fmf42c8vp3fcysvqpkg4zahl1j-0ad-data-0.0.23b-alpha.drv
The following grafts will be made:
   /gnu/store/hjr33zqgx46ldswbqlmvgz7g37l7hrxl-abbaye-2.0.1.drv
   /gnu/store/71rdqs3hldmy1d92742camwc97l3pb3w-7kaa-2.15.3.drv
   /gnu/store/0b92jff2kfmd6l23y8v9xcprk8l7casw-4store-1.1.6.drv
   /gnu/store/82pj69v420fjmkqjxjxix3cckwcfdwg2-4dtris-0.4.3.drv
   /gnu/store/gamc676fdg1p7qzvzwfq7rp6qn4agss1-389-ds-base-1.4.0.21.drv
   /gnu/store/p310ysfwix0ci2594x78vv4x5isqcxix-0ad-0.0.23b-alpha.drv
guix build: error: all build users are currently in use; consider creating 
additional users and adding them to the `guixbuild' group
--8<---------------cut here---------------end--------------->8---

The key is passing more than 10 packages (I have 10 build users) as an
argument when using the '--check' argument.

I'll update the title to reflect the actual bug.

Maxim