Re-enable signing validation

[dougbu]

#13040 will leave us without signing validation for now.

@joeloff is looking into possible bugs in this part of Arcade. Once that is done, we should remove the enableSigningValidation: false line in ci.yml (https://github.com/aspnet/AspNetCore/pull/13040/files#diff-097cdf55e1a931b74fbfe48d7e41c8beR593).

[Pilchie]

Keeping in 3.0, as I don't want to enter servicing without signing validation.