Pixi cpp-fmt command is not portable

[jleibs]

Running produces:

✨ Pixi task (cpp-fmt in cpp): fd --extension h --extension hpp --extension c  --extension cpp --exec clang-format -i
fd: command not found

We need a portable alternative to fd.

[jleibs]

We can just add this to the environment via pixi add fd-find. I'll make a PR once #6560 lands to avoid merge-conflicts on the pixi.lock file.