CI: Add another "acceptance" implementation

[woodruffw]

At the moment, only sigstore-python is only used for 'acceptance' into the conformance suite, meaning that it's pretty easy to merge FP/FN or otherwise incorrect cases because they're either skipped or a single implementation incorrectly accepts them. See #112 for an example of this.

To mitigate this, we could add another self-test with a separate implementation, such as sigstore-go.