Fix inconsistent usage of /bin/sh vs. sh

For now, we mostly use "/bin/sh" for background tasks; but sometimes
we call it as "sh". The former does not work on android, but I'm not
yet sure how we should fix that.

Let's make things consistent first, so they are easier to change
(we might remove some background tasks, or reimplement them in fish
script).

share/functions/__fish_cached.fish

@@ -23,7 +23,7 @@ function __fish_cached --description "Cache the command output for a given amoun
     set -l cache_file (path normalize $cache_dir/$cache_key)
     set -l cache_age (path mtime --relative $cache_file)
 
-    set -l populate_cache sh -c "
+    set -l populate_cache /bin/sh -c "
         {
             $argv
         } >$cache_file || rm $cache_file 2>/dev/null