Is the mitzva of tza'ra'as on the kohein to investigate tumah or on the yisrael to come forward?

What drew me to have a look at that Chinuch was really a different issue. The Ramban asks why there is no pasuk instructing "dabeir el Bnei Yisrael" by tza'ra'as. He answers the kohein was enpowered to be metamei people even withou their consent if he found tumah, and thus the command is addressed to kohanim. However, the Chinuch writes that there is a mitzva to seek out the kohein if you see a nega tza'ra'as. Sounds like a machlokes to me, but in Chavel's notes on the the Ramban he cites the Kli Chemdah who does not think so. Kli Chemdah asks why the Ramban ignores the obligation on the people and answers that since they were not in Eretz Yisrael yet it was inapplicable.