fossil: 2.5 -> 2.6

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/fossil/versions.

These checks were done:

- built on NixOS
- ran ‘/nix/store/by0my4ix1gpichp15gf3i8w3fgfxmsrc-fossil-2.6/bin/fossil --help’ got 0 exit code
- ran ‘/nix/store/by0my4ix1gpichp15gf3i8w3fgfxmsrc-fossil-2.6/bin/fossil help’ got 0 exit code
- found 2.6 with grep in /nix/store/by0my4ix1gpichp15gf3i8w3fgfxmsrc-fossil-2.6
- directory tree listing: https://gist.github.com/673b50672a3bb2c22d7e37175d1f37d4