Make diff-root think NAN == NAN

Unfortunately a number of inputs to the flavor taggers are sometimes NAN. This is fine from a computing and physics point of view: it's reproducible and the downstream code handles these inputs fine. The place where it's not fine is in the frozen derivation tests: our diff script recognizes two files with identical branches as different because NAN != NAN according to IEEE standards.

Since this isn't actually a difference from the regression testing point of view, this commit makes acmd diff-root --nan-equal script respect NAN == NAN. I've also enabled this in the frozen derivation tests.

This is a followup to !28552 (closed), which didn't end up working so well.