zsh: don't load certain plugins when their required binaries don't exist

.zshrc

@@ -71,9 +71,13 @@ ${SKIP_PLUGINS:-false} || {
       fi
 
       # Plugins for use with Nix
+      if command -v nix &> /dev/null; then
          zinit load "chisui/zsh-nix-shell"
          zinit load "spwhitt/nix-zsh-completions"
+      fi
+      if command -v direnv &> /dev/null; then
          zinit snippet OMZP::direnv
+      fi
 
       ## Theme
       # terminal colors