ci: Define OPENSSL_ROOT_DIR on AppVeyor
ci: Define OPENSSL_ROOT_DIR on AppVeyor

On Appveyor, Windows Server 2019 is equipped with several OpenSSL
libraries.

   https://www.appveyor.com/docs/windows-images-software/

We need to specify the exact OpenSSL path to link against the latest
OpenSSL versions.

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@ceptord.net>

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