bug#41602: texlive-texmf is actually subtitutable

[Ludovic Courtès]

Strangely, ‘texlive-texmf’ (the big one) is substitutable:

--8<---------------cut here---------------start------------->8---
$ guix describe
Generacio 145   May 25 2020 00:37:58    (nuna)
  guix 9744cc7
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 9744cc7b4636fafb772c94adb8f05961b5b39f16
$ guix environment --ad-hoc texlive -- texdoc biblatex
2.6 MB will be downloaded:
   /gnu/store/7ji4l3szj68b0r5w10bvvdx1vy6nhz5p-subversion-1.10.6
downloading from 
https://ci.guix.gnu.org/nar/lzip/7ji4l3szj68b0r5w10bvvdx1vy6nhz5p-subversion-1.10.6
 ...
 subversion-1.10.6  2.5MiB                                                      
                                                                          
7.2MiB/s 00:00 [##################] 100.0%

La jena derivo estos konstruata:
   /gnu/store/55yx02hr0dz47px1aj0j14xll3bsrmml-texlive-texmf-20190410.drv
2,845.8 MB will be downloaded:
   /gnu/store/nm6w84c9zj3yiylal3dk1sqzxq11sjzw-texlive-20190410-texmf.tar.xz
   /gnu/store/xpkl70g3bls935h1zdlq7sn2j6rccp3k-texlive-20190410
downloading from 
https://ci.guix.gnu.org/nar/lzip/z4xvgiliw5baf1pr4z03c7n2hw3bm5x5-texlive-texmf-20190410
 ...
 texlive-texmf-20190410  2.61GiB
--8<---------------cut here---------------end--------------->8---

The info suggests it won’t be substituted, but it’s eventually
substituted.  I wonder why, because the .drv has:

  ("allowSubstitutes","0")

and the daemon has:

  bool substitutesAllowed(const Derivation & drv)
  {
      return get(drv.env, "allowSubstitutes", "1") == "1";
  }

and:

  if (settings.useSubstitutes && substitutesAllowed(drv))
      foreach (PathSet::iterator, i, invalidOutputs)
          addWaitee(worker.makeSubstitutionGoal(*i, buildMode == bmRepair));

Thoughts?

Ludo’.