did:x509 validation of key usage is incorrect

[reinkrul]

During the LSPxNuts hackathon, we tried to load a VC issued by a UZI smartcard, with the signing certificate. Our invocation of x509.Verify is incorrect in this case, because it provides "server cert" as usage (which is the default when nothing else is specified).

The following error occurs:

could not validate issuer: did:509 certificate chain validation failed: x509: certificate specifies an incompatible key usage