attestion: make InvalidAttestationError non-fatal in CI

I don't think I've seen an `InvalidAttestationError` that wasn't some
sort of network problem (e.g., rate limit, connection timeout, 503).
Let's emit a warning instead of erroring out.

Note that `MissingAttestationError` is still fatal, and that will still
produce errors in CI.

Library/Homebrew/attestation.rb

@@ -246,6 +246,12 @@ module Homebrew
       end
 
       backfill_attestation
+    rescue InvalidAttestationError => e
+      raise if ENV["HOMEBREW_GITHUB_ACTIONS"].blank?
+
+      opoo "Attestation verification failed (please verify that this is not a network error before rebottling): #{e}"
+
+      {}
     end
   end
 end