Fix double slash in index path (#775)

* Fix double slash in index path

Double slash might disturb comparison and make them false positive.

* Fix compatibility with older fish shell

Suggested by @faho [here](https://github.com/oh-my-fish/oh-my-fish/pull/775#issuecomment-736224485).

pkg/omf/functions/index/omf.index.path.fish

 function omf.index.path -d 'Get the path to the local package index'
   set -q XDG_CACHE_HOME
-    and echo "$XDG_CACHE_HOME/omf"
-    or echo "$HOME/.cache/omf"
+    and echo (echo $XDG_CACHE_HOME | sed 's:/*$::')"/omf"
+    or echo (echo $HOME | sed 's:/*$::')"/.cache/omf"
 end