Fix unencrypted linux posture CI test

Our Linux posture CI job uses an AWS CodeBuild GitHub Actions runner configured as:
- EC2
- Instance mode, not Docker
- Managed Amazon Linux image
- aws/codebuild/ami/amazonlinux-x86_64-base:latest

The runner has several compatibility problems:
1. Old glibc
- Managed image provides glibc older than required by GitHub Actions’ Node runtimes.
- actions/checkout fails with:
GLIBC_2.27 not found
GLIBC_2.28 not found
- This affects both checkout@v6/Node 24 and checkout@v3/Node 20.
2. OpenSSL incompatibility
- The image provides OpenSSL 1.0.2.
- Current Rust dependencies require OpenSSL 1.1+:
openssl-sys ... This crate is only compatible with OpenSSL 1.1.0, 1.1.1, 3.x, or 4.x
3. No newer managed Linux image appears available
- The CodeBuild runner UI only exposes Amazon Linux and Windows Server.
- The available Amazon Linux image choices do not provide a newer glibc environment.

**Impact**
The managed CodeBuild runner cannot reliably execute the Linux posture CI workflow. We currently need workarounds for repository checkout and dependency installation.

**Desired Outcome**
Determine the supported AWS or self-hosted architecture for this CI job:
- A CodeBuild image with newer glibc and OpenSSL support.
- A supported way to use a custom EC2/AMI image.
- Or a dedicated self-hosted EC2/own infra runner with a controlled Linux system.