[Backport release-21.11] umap-learn: fix build

[github-actions]

Bot-based backport to release-21.11, triggered by a label in #147662.

• Before merging, ensure that this backport complies with the Criteria for Backporting.
  • Even as a non-commiter, if you find that it does not comply, leave a comment.

[risicle]

ZHF #144627

[risicle]

Hold on, umap-learn is already successfully building on 21.11: https://hydra.nixos.org/eval/1726172?filter=umap-learn&compare=1725962&full=

[veprbl]

It was broken in update to tensorflow 2.7.0, which is not in 21.11. I assume, that previously tensorflow would ship a copy of keras, and just "depends" on it. In nixpkgs, currently, we don't have keras in tensorflow.propagatedBuildInputs, so it needs to be added explicitly as needed.