fix: nightly Linux build fails — openssl-sys not found

[noahgift]

Five-Whys Root Cause

1. Why does nightly fail? error: failed to run custom build command for openssl-sys
2. Why can't openssl-sys build? The system library openssl was not found
3. Why isn't openssl installed? The nightly workflow doesn't install libssl-dev before building
4. Why isn't it needed on other builds? macOS has openssl via brew/system; Windows uses different TLS
5. Why did this start failing? pforge added a dependency chain that pulls in openssl-sys

Fix

Add sudo apt-get install -y libssl-dev pkg-config before the Linux build step.
Or use cross which includes openssl in its build container.

Discovered by kaizen sweep 2026-04-06. Refs paiml/infra#20.