Update is_govuk_team? method

In #570, we added the ability to
post alerts to a different Slack channel to the main team.

Changing the hash key in this method was missed.

lib/team_builder.rb

@@ -103,6 +103,6 @@ def govuk_team_repos(team_channel)
   end
 
   def is_govuk_team?(team)
-    @govuk_data.any? { |repo| repo["team"] == "##{team}" }
+    @govuk_data.any? { |repo| repo["alerts_team"] == "##{team}" }
   end
 end