treewide: add meta.mainProgram

Related PR: https://github.com/NixOS/nixpkgs/pull/246386

pkgs/applications/networking/nload/default.nix

···
     license = lib.licenses.gpl2;
     platforms = lib.platforms.linux;
     maintainers = [ lib.maintainers.devhell ];
+    mainProgram = "nload";
   };
 }

pkgs/shells/any-nix-shell/default.nix

···
     license = licenses.mit;
     homepage = "https://github.com/haslersn/any-nix-shell";
     maintainers = with maintainers; [ haslersn ];
+    mainProgram = "any-nix-shell";
   };
 }

pkgs/shells/zsh/default.nix

···
     homepage = "https://www.zsh.org/";
     maintainers = with lib.maintainers; [ pSub artturin ];
     platforms = lib.platforms.unix;
+    mainProgram = "zsh";
   };
 
   passthru = {

pkgs/tools/admin/procs/default.nix

···
     changelog = "https://github.com/dalance/procs/raw/v${version}/CHANGELOG.md";
     license = licenses.mit;
     maintainers = with maintainers; [ Br1ght0ne sciencentistguy ];
+    mainProgram = "procs";
   };
 }

pkgs/tools/misc/exa/default.nix

···
     homepage = "https://the.exa.website";
     license = licenses.mit;
     maintainers = with maintainers; [ ehegnes lilyball globin fortuneteller2k ];
+    mainProgram = "exa";
   };
 }