gauche: set meta.mainProgram to gosh

This package provides multiple binaries, but `gosh' is the
interpreter which is actually used to run scheme scripts,
and so probably the one people think of when they say
"running Gauche Scheme".

pkgs/development/interpreters/gauche/default.nix

···
   meta = with lib; {
     description = "R7RS Scheme scripting engine";
     homepage = "https://practical-scheme.net/gauche/";
+    mainProgram = "gosh";
     maintainers = with maintainers; [ mnacamura ];
     license = licenses.bsd3;
     platforms = platforms.unix;