tdf#161872 xmlsecurity nss: don't require trusted signing certs

Configure your signing cert in Firefox, (unusually) don't trust the CA
that you would use to issue your signing cert, try to sign a document,
error.

This is reportedly wrong since commit
bfd479abf0 (Update libxmlsec to 1.3.1,
2023-06-09), for some reason our usage of libxmlsec-1.2 didn't trigger
this problem.

Given that we already disable libxmlsec-side cert verify while
verifying a signature (and have our own logic there), it's consistent to
do the same while creating the signature, and that fixes the bug, too.

The Windows / MSCNG backend is not yet changed here.

Change-Id: I8a7adf06b9a26731f1a180a4f6257317084e414a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/180844
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

xmlsecurity/source/xmlsec/nss/xmlsignature_nssimpl.cxx

@@ -150,6 +150,7 @@ SAL_CALL XMLSignature_NssImpl::generate(
     }
 
     //Sign the template
+    pDsigCtx->keyInfoReadCtx.flags |= XMLSEC_KEYINFO_FLAGS_X509DATA_DONT_VERIFY_CERTS;
     if( xmlSecDSigCtxSign( pDsigCtx.get() , pNode ) == 0 )
     {
         if (pDsigCtx->status == xmlSecDSigStatusSucceeded)