migrate some more code to mu_format / join_paths

Let's modernize a bit.

mu/mu-cmd-cfind.cc

@@ -95,7 +95,7 @@ guess_nick(const Contact& contact)
 		nicks.emplace(nick, 0);
 	else {
 		++it->second;
-		nick = format("%s%zu", nick.c_str(), ++it->second);
+		nick = mu_format("{}{}", nick, ++it->second);
 	}
 
 	return nick;

mu/mu-cmd-extract.cc

@@ -38,7 +38,7 @@ save_part(const Message::Part& part, size_t idx, const Options& opts)
 	 * cooking */
 	const auto path{targetdir +
 		part.cooked_filename(opts.extract.uncooked)
-		.value_or(format("part-%zu", idx))};
+		.value_or(mu_format("part-{}", idx))};
 
 	if (auto&& res{part.to_file(path, opts.extract.overwrite)}; !res)
 		return Err(res.error());