Fixes <https://bugs.gnu.org/41400>.
Reported by Mikhail Tsykalov <tsymsh@gmail.com>.
This essentially reverts 40fd909e3d.
This is a followup to the TeX Live 2019 update in
85892948bd.
* gnu/packages/tex.scm (biber): Update to 2.12.
Biber 2.12 is incompatible with biblatex 3.11, which is what's currently
packaged. This is a followup to 41a010875b.
* gnu/packages/tex.scm (biber): Downgrade to 2.11.