test-bot: cleanup Cellar/etc/var after each build.

This has been causing issues when bottling files that are installed to
these paths due to the diff algorithm we use for figuring out whether
files installed into etc/var belong to a given bottle. Additionally,
sometimes not all formulae were uninstalled which meant `brew doctor`
would fail.

Library/Homebrew/dev-cmd/test-bot.rb

@@ -749,7 +749,7 @@ def homebrew
       end
     end
 
-    def cleanup_git
+    def cleanup_shared
       git "gc", "--auto"
       test "git", "clean", "-ffdx", "--exclude=Library/Taps"
 
@@ -759,8 +759,8 @@ def cleanup_git
         safe_system "brew", "untap", tap
       end
 
-      Formula.installed.each do |formula|
-        safe_system "brew", "uninstall", "--force", formula
+      Dir.glob("#{HOMEBREW_PREFIX}/{Cellar,etc,var}/**/*").each do |file|
+        FileUtils.rm_rf file
       end
       safe_system "brew", "prune"
 
@@ -792,7 +792,7 @@ def cleanup_before
         git "reset", "--hard", "origin/master"
       end
 
-      cleanup_git
+      cleanup_shared
 
       pr_locks = "#{@repository}/.git/refs/remotes/*/pr/*/*.lock"
       Dir.glob(pr_locks) { |lock| FileUtils.rm_rf lock }
@@ -813,7 +813,7 @@ def cleanup_after
         git "stash", "pop"
         test "brew", "cleanup", "--prune=7"
 
-        cleanup_git
+        cleanup_shared
 
         if ARGV.include? "--local"
           FileUtils.rm_rf ENV["HOMEBREW_HOME"]