Try reverting the only changes to build_doc since the break

[PProfizi]

The Documentation check passes but the documentation artifact is actually empty (easily verified with its small size of 48 kB, should be more than 100MB).
The last known Documentation generation job to work was here on August 22.
Was rerun to check for unseen change in a dependency, it still passed.

Seems to have started with bump of Graphviz to 0.20.1 here. Cannot find an occurrence before that.

Two problems here:

the documentation check did not see something was wrong with the doc (artifact too small / step too short)
no error was raised although something went obviously wrong

[codecov]

Codecov Report

Merging #429 (b767277) into master (3ca6c70) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #429      +/-   ##
==========================================
- Coverage   86.39%   86.31%   -0.08%     
==========================================
  Files          61       61              
  Lines        6717     6717              
==========================================
- Hits         5803     5798       -5     
- Misses        914      919       +5