signal-desktop: 1.6.1 -> 1.7.0

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

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

These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 1.7.0 with grep in /nix/store/xkslw2sdzbpbzy0ddd0f5qxk1wkfahs6-signal-desktop-1.7.0
- directory tree listing: https://gist.github.com/05c86387ecf6ea0334f3d0c18b44542e