See description of QgsAuthCertUtils::pkcs8PrivateKey.
This fix may be needed on other platforms (untested at this point),
because Qt5 QSslkey class *still* does not directly support creation
using non-PKCS1 PEM- or DER-encoded data, though QCA, whose qca-ossl
plugin is linked to OpenSSL, does support PKCS1 and PKCS8.
