nixos/mautrix-telegram: loosen umask to keep `config.json` writable
nixos/mautrix-telegram: loosen umask to keep `config.json` writable

This is needed because `mautrix-telegram --generate-registration`
appears to need write-access to `config.json` as well.

Closes #135884

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs