Prefer eval to source for direnv in config.fish

Calling `source` on `(direnv hook fish)` was creating startup problems with
fish. These problems leaked into a few of my Emacs file-searching commands as
well, which was pretty irritating for awhile. I'm still unsure of the
differences between `eval` and `source`. I'm moving on for now.

configs/shared/.config/fish/config.fish

@@ -87,9 +87,7 @@ source (fzf-share)/key-bindings.fish && fzf_key_bindings
 # TODO: Debug why I couldn't get `fasd` to work with Nix and fish.
 source (nix_find autojump)/share/autojump/autojump.fish
 
-# TODO: What is the difference between `source` and `eval`
-# direnv
-source (direnv hook fish)
+eval (direnv hook fish)
 
 # Miscellaneous
 abbr --add c xclip -selection clipboard -i