Use the cache directory owner for cache entries

Commit f086bc9564 (Maintain ownership when rewriting universal variables
file, 2015-09-26) fixed an issue where "sudo -E fish" would create root-owned
~/.config/fish/fish_variables that break the users's shell.

A simlar issue exists for files in ~/.cache/fish; fix that.

Note that ~/.cache/fish is currently created on first run when we generate
completions from manpages.

See the issue described in #11292

share/functions/__fish_cache_put.fish

@@ -0,0 +1,10 @@
+function __fish_cache_put
+    set -l cache_file $argv[1]
+    touch $cache_file
+    set -l dir (path dirname $cache_file)
+    chown --reference=$dir $cache_file 2>/dev/null ||
+        chown (
+            stat --format '%u:%g' $dir 2>/dev/null ||
+            stat -f '%u:%g' $dir
+        ) $cache_file
+end