Motivation for this change

By default, the Insomnia package adds a .desktop icon with an Exec path of "/opt/insomnia/insomnia", but that's (generally!) not where Nix installs Insomnia.

Things done

I added a sed into the "installPhase" to replace the "/opt" path with the "$out" path.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.