GitHub Actions: add other config flavours

GitHub Actions: add other config flavours

In the past we had issues with patches accidentally breaking less common
OpenVPN configurations. For this reason it makes sense to some of those
configurations build via CI.

Adding:
* --enable-iproute2
* --enable-async-push
* --disable-management
* --enable-small
* --disable-lzo --disable-lz4

These flavours are all built on Ubuntu 20.04 with OpenSSL 1.1.1

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220121133900.14567-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23624.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>