Fix error with conda-libmamba-solver (libarchive.so.19: cannot open shared object file).

Remove legacy definition of `ENV` variables.

Use `git mv` instead of `mv` to rename the file(s).
This ensure `git` is aware of the renaming, otherwise you might move to the next step (`nbconvert`) with the staged files containing both the original file name(s) and renamed file(s)!