treewide: add `meta.mainProgram`

Add `meta.mainProgram` to all packages for which lib.getExe is called in
home-manager

pkgs/applications/misc/batsignal/default.nix

···
     license = licenses.isc;
     maintainers = with maintainers; [ SlothOfAnarchy ];
     platforms = platforms.linux;
+    mainProgram = "batsignal";
   };
 }

pkgs/applications/misc/safeeyes/default.nix

···
     license = licenses.gpl3;
     maintainers = with maintainers; [ srghma ];
     platforms = platforms.linux;
+    mainProgram = "safeeyes";
   };
 }

pkgs/development/tools/pyenv/default.nix

···
     license = licenses.mit;
     maintainers = with maintainers; [ tjni ];
     platforms = platforms.all;
+    mainProgram = "pyenv";
   };
 }

pkgs/tools/misc/rtx/default.nix

···
     changelog = "https://github.com/jdxcode/rtx/releases/tag/v${version}";
     license = lib.licenses.mit;
     maintainers = with lib.maintainers; [ konradmalik ];
+    mainProgram = "rtx";
   };
 }

pkgs/tools/networking/goimapnotify/default.nix

···
     homepage = "https://gitlab.com/shackra/goimapnotify";
     license = licenses.gpl3Plus;
     maintainers = with maintainers; [ wohanley ];
+    mainProgram = "goimapnotify";
   };
 }