os/linux/diagnostic: skip error on arm64 when HOMEBREW_ARM64_TESTING is set

This[^1] will enable us to start testing arm64 bottle builds in
Homebrew/core when this environment variable is set.

[^1]: Along with some tweaks to the `dispatch-build-bottle` workflow.

Library/Homebrew/extend/os/linux/diagnostic.rb

@@ -74,7 +74,8 @@ module OS
         end
 
         def check_supported_architecture
-          return if Hardware::CPU.arch == :x86_64
+          return if Hardware::CPU.intel?
+          return if Homebrew::EnvConfig.developer? && ENV["HOMEBREW_ARM64_TESTING"].present? && Hardware::CPU.arm?
 
           <<~EOS
             Your CPU architecture (#{Hardware::CPU.arch}) is not supported. We only support