cpu.rb: clean up the list of CPU instructions

Co-authored-by: Shaun Jackman <sjackman@gmail.com>
Co-authored-by: Markus Reiter <me@reitermark.us>

Library/Homebrew/extend/os/linux/hardware/cpu.rb

@@ -69,8 +69,9 @@ module Hardware
         end
       end
 
+      # Supported CPU instructions
       def flags
-        @flags ||= cpuinfo[/^(flags|Features).*/, 0]&.split
+        @flags ||= cpuinfo[/^(?:flags|Features)\s*: (.*)/, 1]&.split
         @flags ||= []
       end