Using an ECDSA key but writing RSA URIs would fail later in libxmlsec.

Also fix up CppunitTest_xmlsecurity_signing (env vars were set too
late), so that the new testcase actually fails without the fix.

Change-Id: I9e584844d5cd046952b2f19130aeaa5a765bfc0a
Reviewed-on: https://gerrit.libreoffice.org/54400Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>