fix: set PKG_CONFIG_PATH for openssl in CI workflow
nixery containers don't automatically set PKG_CONFIG_PATH like nix-shell does.
dynamically find the openssl.dev path in /nix/store and set PKG_CONFIG_PATH
to point to its pkgconfig directory before running cargo commands.
tested locally in nixos/nix docker container with the same dependencies.
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Co-Authored-By: Claude <noreply@anthropic.com>