You can use either forming or building when describing friendships or relationships in general. Forming friendships generally means that you weren't exactly friends with the person before but youve now formed a friendship with them. Building friendships also has the same meaning but has the connotation that youre actively improving your relationship with them. Forming trust/friendships/relationships=none there but now youre making it. Building trust/friendships/relationships=maybe it was there maybe it wasn't but youre improving it.