zsh: restore the vendored fzf PATH entry, guarded

The hygiene pass (e960eda) dropped path=(~/.cache/lk-tools/fzf/bin $path) as
stale. On Linux boxes set up by lk-installers/install-apps it is the only thing
that puts fzf on PATH (install-apps vendors fzf there and relies on .zshrc to
add it), so fzf, fzf-tab and fzf-git broke in new shells. Restored behind an
existence guard, matching that commit's only-if-installed style, so boxes
without the vendored copy are unaffected.

home_root/.zshrc

@@ -1,6 +1,8 @@
 #extend path
 path=(~/bin $path)
 path=(~/.local/bin $path)
+# fzf vendored by lk-installers/install-apps (only if installed)
+[ -d ~/.cache/lk-tools/fzf/bin ] && path=(~/.cache/lk-tools/fzf/bin $path)
 
 # Enable Powerlevel10k instant prompt. Should stay close to the top of ~/.zshrc.
 # Initialization code that may require console input (password prompts, [y/n]