search_remote_tap spec: fix test offline

Even though we stub the JSON response, the method being tested always
returns [] immediately if HOMEBREW_NO_GITHUB_API is set.

Library/Homebrew/test/cmd/search_remote_tap_spec.rb

@@ -2,6 +2,9 @@ require "cmd/search"
 
 describe Homebrew do
   specify "#search_taps" do
+    # Otherwise the tested method returns [], regardless of our stub
+    ENV.delete("HOMEBREW_NO_GITHUB_API")
+
     json_response = {
       "items" => [
         {