Use /Library/Caches/Homebrew

We don't penalise existing users; if ~/Library/Caches/Homebrew already exists and is writable, we select that.

This is the correct choice, the cache should be the same directory whichever user is used and whatever instance of brew is invoked.

The Cache directory is set to 0777 this allows any user to write to it and fixes Homebrew/homebrew#10857.

Library/Homebrew/global.rb

@@ -13,22 +13,29 @@ def cache
   if ENV['HOMEBREW_CACHE']
     Pathname.new(ENV['HOMEBREW_CACHE'])
   else
-    root_library = Pathname.new("/Library/Caches/Homebrew")
-    if Process.uid == 0
-      root_library
+    # 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
+    home_cache = Pathname.new("~/Library/Caches/Homebrew").expand_path
+    if home_cache.directory? and home_cache.writable?
+      home_cache
     else
-      home_library = Pathname.new("~/Library/Caches/Homebrew").expand_path
-      if not home_library.writable?
-        root_library
-      else
-        home_library
+      root_cache = Pathname.new("/Library/Caches/Homebrew")
+      class << root_cache
+        alias :oldmkpath :mkpath
+        def mkpath
+          unless exist?
+            oldmkpath
+            chmod 0777
+          end
+        end
       end
+      root_cache
     end
   end
 end
 
 HOMEBREW_CACHE = cache
-undef cache
+undef cache # we use a function to prevent adding home_cache to the global scope
 
 # Where brews installed via URL are cached
 HOMEBREW_CACHE_FORMULA = HOMEBREW_CACHE+"Formula"