terra: set meta.platforms

Upstream claims MacOS, Linux, and Windows as supported platforms; set to unix
for now.

pkgs/development/compilers/terra/default.nix

@@ -60,6 +60,7 @@ stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     inherit (src.meta) homepage;
     description = "A low-level counterpart to Lua";
+    platforms = platforms.unix;
     maintainers = with maintainers; [ jb55 ];
     license = licenses.mit;
   };