[CI] conda_build step is taking a long time

[lasofivec]

Since last update of conda-build, the time of the building library went up to 18 mins.

[lasofivec]

It seems it is coming from conda/conda-build#3775 (comment)

[Didou09]

good catch, indeed it seems conda-dependent

[Didou09]

Closed since PR #249 which removed two nested for loops in tests01_geom/tests03_core.Test03_Rays.calc_signal()