sha1collisiondetection: set meta.mainProgram

Sets `meta.mainProgram = "sha1dcsum"`.

This lets `nix run nixpkgs#sha1collisiondetection` do something useful,
namely run `sha1dcsum`.

pkgs/by-name/sh/sha1collisiondetection/package.nix

@@ -39,5 +39,6 @@ stdenv.mkDerivation rec {
     platforms = platforms.all;
     maintainers = with maintainers; [ leenaars ];
     license = licenses.mit;
+    mainProgram = "sha1dcsum";
   };
 }