Add `meta.mainProgram` (#252060)

pkgs/applications/virtualization/tart/default.nix

···
     homepage = "https://tart.run";
     license = licenses.fairsource09;
     maintainers = with maintainers; [ emilytrau Enzime ];
+    mainProgram = finalAttrs.pname;
     platforms = [ "aarch64-darwin" ];
     sourceProvenance = with sourceTypes; [ binaryNativeCode ];
   };

pkgs/development/python-modules/vncdo/default.nix

···
 , nose
 , ptyprocess
 }:
+
 buildPythonPackage rec {
   pname = "vncdo";
   version = "0.12.0";
···
     description = "A command line VNC client and python library";
     license = licenses.mit;
     maintainers = with maintainers; [ elitak ];
+    mainProgram = pname;
     platforms = with platforms; linux ++ darwin;
   };
-
 }

pkgs/tools/networking/passh/default.nix

···
 { lib, fetchFromGitHub, stdenv }:
-stdenv.mkDerivation rec {
+
+stdenv.mkDerivation (finalAttrs: {
   pname = "passh";
   version = "2020-03-18";
 
   src = fetchFromGitHub {
     owner = "clarkwang";
-    repo = pname;
+    repo = finalAttrs.pname;
     rev = "7112e667fc9e65f41c384f89ff6938d23e86826c";
     sha256 = "1g0rx94vqg36kp46f8v4x6jcmvdk85ds6bkrpayq772hbdm1b5z5";
   };
 
   installPhase = ''
     mkdir -p $out/bin
-    cp passh $out/bin
+    cp ${finalAttrs.pname} $out/bin
   '';
 
   meta = with lib; {
···
     description = "An sshpass alternative for non-interactive ssh auth";
     license = licenses.gpl3;
     maintainers = [ maintainers.lovesegfault ];
+    mainProgram = finalAttrs.pname;
     platforms = platforms.unix;
   };
-}
+})