Link against libcurl (openssl) instead of libcurl-gnutls in Linux

[pol-rivero]

The newest version of GitHub Desktop breaks in Fedora, RHEL and CentOS due to upgrading dugite-native from 2.47.1 to 2.47.3.
The previous version linked against libcurl.so.4, while the new one links against libcurl-gnutls.so.4 which is not available in those distros.

This was already fixed in #102, until this regression. We suspect the regressor is this commit: c64f13f because it begins using ubuntu's distro-specific curl instead of building it. However, we have not tested if that's the case.

Please let me know if you need more information.