Append (not prepend) bin directory to PATH

Prepending can be problematic when the user install fzf using Homebrew,
execute the install script, and later upgrade fzf with Homebrew, and do
not rerun the install script. In that case, even though the homebrew
package is upgraded, the older version will still be used.

install

@@ -181,7 +181,7 @@ for shell in bash zsh; do
 # Setup fzf
 # ---------
 if [[ ! "\$PATH" =~ "$fzf_base/bin" ]]; then
-  export PATH="$fzf_base/bin:\$PATH"
+  export PATH="\$PATH:$fzf_base/bin"
 fi
 
 # Man path