cargo: Ensure libssh2 from system is used
cargo: Ensure libssh2 from system is used

This forces the use of libssh2 by exporting the
LIBSSH2_SYS_USE_PKG_CONFIG environment variable.

Signed-off-by: Otavio Salvador <1d31b11c1b92bf5d043d36edfe7174db39a6b891@ossystems.com.br>

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