runners: bake step-ca root CA into image (v20260525-stepca)

Without the IAmWorkin step-ca root CA in the runner image's system
trust store, .NET HttpClient calls from CI tests against
`*.iamworkin.lan` (e.g. `https://selenium.iamworkin.lan/session`) fail
with `The remote certificate is invalid because of errors in the
certificate chain: PartialChain`. FlowerCore.Print.Web's
`WebScreenshotService` unit tests hit this on every build.

Drop the step-ca root PEM into `/usr/local/share/ca-certificates/`,
run `update-ca-certificates` once during apt install, and let OpenSSL +
.NET-on-Linux read the regenerated `/etc/ssl/certs/ca-certificates.crt`
automatically — no `SSL_CERT_FILE` env var, no per-Deployment volume
mount.

Image rebuilt + saved + imported on all 3 schedulable RKE2 nodes
(rke2-server, rke2-agent1, rke2-agent2) before this PR — verified with
`ctr images list -q | grep stepca` on each node.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/github-runner/step-ca-root.crt

@@ -0,0 +1,12 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----