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

[Maxim Cournoyer]

Hello,

While working on ruby-asciidoctor-pdf, the following command failed
because of maxing out build users:

./pre-inst-env guix build --quiet --keep-going --check ruby_version@1.0.2 
gnome-shell-extension-gsconnect@33 numix-gtk-theme@2.6.7 arc-theme@20190917 
gnome@3.34.2 ruby-fakefs@1.2.2 ruby-protobuf@3.10.3 ruby-fast-gettext@2.0.3 
ruby-sass-rails@5.0.7 mailcatcher@0.7.1 ruby-rspec-its@1.3.0 warzone2100@3.4.0 
keepassxc@2.6.0 wsjtx@2.2.2 js8call@2.2.0 ruby-yard-with-tests@0.9.25 
ruby-asciidoctor-pdf@1.5.3-1.f25ea78 ruby-web-console@3.7.0 
ruby-autoprefixer-rails@9.4.7 ruby-jekyll-paginate-v2@2.0.0 
ruby-solargraph@0.36.0 --max-jobs=1
The following derivations will be built:
   /gnu/store/w5qba9nzswv7ysrlxfwcqdvrf9vsgbk6-ruby-jekyll-paginate-v2-2.0.0.drv
   /gnu/store/4r1ziw3cb5n5jgknd1jfsvdl46cz56f9-ruby-autoprefixer-rails-9.4.7.drv
   /gnu/store/2f674244hhb399p86z15zgv246sj78gr-ruby-web-console-3.7.0.drv
   /gnu/store/sqy6f63vzgbyk6jg1zigxvks9g365wfs-ruby-rspec-its-1.3.0.drv
   /gnu/store/zz34dh2q37r21r2d26k7z5rpryysc3km-ruby-sass-rails-5.0.7.drv
   /gnu/store/62nlbfwd2jf01nm0p0wlc9sgvn5yp6gc-ruby-fakefs-1.2.2.drv
   /gnu/store/pjdh08jh7pxlfm4dcz86bg5xqbjyf4lm-gnome-3.34.2.drv
   /gnu/store/xqdvar6gmkz28003md4w9c1mqvg0slas-arc-theme-20190917.drv
   /gnu/store/qwmj9b493ays4dvgkmniq61fnxg3claw-numix-gtk-theme-2.6.7.drv
   /gnu/store/s112ayw6qg86y7j45r5yslfl151ngvgn-ruby_version-1.0.2.drv
The following grafts will be made:
   /gnu/store/ww8blyc1nkf788jjb3anh22dylibvbah-ruby-solargraph-0.36.0.drv
   
/gnu/store/c7bfyqfq8d7imgy8il7wzqpx3qpx4zpw-ruby-asciidoctor-pdf-1.5.3-1.f25ea78.drv
   /gnu/store/cf501w2kdgw6k2gv6n24ikhzww8f51x7-ruby-yard-with-tests-0.9.25.drv
   /gnu/store/da8hhcaj20z06f2kww0sg4y24ma034dy-js8call-2.2.0.drv
   /gnu/store/zn1jkg86fpji54s0pa0lfnf1dy00s2nh-wsjtx-2.2.2.drv
   /gnu/store/77g250mf14rskilpsggn070y4fjrbngx-keepassxc-2.6.0.drv
   /gnu/store/5k393vfm5qw442ya7v061q5fmjvhhcs5-warzone2100-3.4.0.drv
   /gnu/store/5d4h76qsr0rmbkn2c0zilyjr7mpzk5pd-mailcatcher-0.7.1.drv
   /gnu/store/mxm64sm569fyhdmxh6jlvdl18fwdb6f6-ruby-fast-gettext-2.0.3.drv
   /gnu/store/wglga3r1j6x87hqksrh5d1ra70kl729l-ruby-protobuf-3.10.3.drv
   
/gnu/store/73nw9dxmmp8jykhmzwdpc3wk7mn81yal-gnome-shell-extension-gsconnect-33.drv
guix build: error: all build users are currently in use; consider
creating additional users and adding them to the `guixbuild' group

It seems that grafts trigger the problem, as passing the --no-grafts
option works around it.  Could it be that grafts can't somehow be queued
like normal builds, adapting to fit to the available build slots?

Maxim