Call expand_path on the value of HOMEBREW_CACHE

Fixes Homebrew/homebrew#40752.

Library/Homebrew/config.rb

 def cache
-  if ENV['HOMEBREW_CACHE']
-    Pathname.new(ENV['HOMEBREW_CACHE'])
+  if ENV["HOMEBREW_CACHE"]
+    Pathname.new(ENV["HOMEBREW_CACHE"]).expand_path
   else
     # we do this for historic reasons, however the cache *should* be the same
     # directory whichever user is used and whatever instance of brew is executed