Fix pre-push formatting checks

[joinemm]

Current main branch does not pass nix flake check as the formatting is wrong in prbuilder configuration. This passed our automatic checks because the pre-push task silently reformats the files before running nix flake check. Thus the pre-push checks pass even though files have been changed.

nix fmt has no --check option so this was changed to alejandra --check . nix fmt -- --fail-on-change.

For tasks.py, added some commas so black will format the aliases consistently in the same way. Without this the wrapping would depend on the line length. Imports have also been sorted with isort.