attestion: make InvalidAttestationError non-fatal in CI

[carlocab]

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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.

[carlocab]

attestation.rb:135: Method `system_command!` does not exist on `T.class_of(Homebrew::Attestation)` https://srb.help/7003
     135 |        result = system_command!(gh_executable, args: cmd,
                           ^^^^^^^^^^^^^^^
Errors: 1

Huh, interesting. Not even code I touched though. Would love a suggestion here. CC @dduugg ?

[MikeMcQuaid]

I appreciate the intent here but 👎🏻 as-is.

If we're going to make this non-fatal it feels like it defeats the entire point of having attestation.

I'd like to have us try a couple of retries with an exponential backoff before we do this.