Ensure readall runs without API.

Library/Homebrew/cmd/readall.rb

@@ -38,6 +38,7 @@ module Homebrew
   def readall
     args = readall_args.parse
 
+    Homebrew.with_no_api_env do
       if args.syntax? && args.no_named?
         scan_files = "#{HOMEBREW_LIBRARY_PATH}/**/*.rb"
         ruby_files = Dir.glob(scan_files).grep_v(%r{/(vendor)/})
@@ -56,7 +57,7 @@ module Homebrew
           raise UsageError, "`brew readall` needs a tap or `--eval-all` passed or `HOMEBREW_EVAL_ALL` set!"
         end
 
-      Tap
+        Tap.select(&:installed?)
       else
         args.named.to_installed_taps
       end
@@ -65,4 +66,5 @@ module Homebrew
         Homebrew.failed = true unless Readall.valid_tap?(tap, **options)
       end
     end
+  end
 end