nix functions in .bashrc no longer have explicit -f arguments because NIX_PATH is used to change the search location instead.
Revert "nix functions in .bashrc no longer have explicit -f arguments because NIX_PATH is used to change the search location
instead."

This reverts commit bbd8a1a514b6064fd587c194fb3c51276d2d00b3.

Turns out NIX_PATH is not used for nix-env, so explicit file paths make more sense to use because they are more robust.
More tweaks.  Needed to use nix-defexpr to get nix-env to work with my local distribution
Added code to set up .nix-defexpr the way I want it to be at startup.