You can get logged in user by a GET request.

    $.ajax({
            async: false,
            headers: { "accept": "application/json; odata=verbose" },
            method: "GET",
            url: formCtx.webAttributes.WebUrl + "/_api/web/CurrentUser",
            success: function (data) {
                currentUser = data.d;
            }
        });

Now use `currentUser` in your CSR code. You may omit async if you can handle it properly. 

`_spPageContextInfo.userLoginName` will give you the current user's login name. So you can use it inside your CSR code if you need the login name only.