fix: build from checked-out source, not stale desktop copy

[Akshat-Raj]

No description provided.

[Copilot]

Pull request overview

This PR fixes the ShieldCI GitHub Actions workflow to build and run from the checked-out source in $GITHUB_WORKSPACE instead of a stale copy assumed to exist at $HOME/Desktop/ShieldCI. It also moves hardcoded API credentials to GitHub Actions secrets and adds new steps for building the Kali Docker image and installing test dependencies.

Changes:

• Replaced all $HOME/Desktop/ShieldCI path references with $GITHUB_WORKSPACE, ensuring the workflow operates on the checked-out repository source.
• Moved SHIELDCI_API_URL and SHIELDCI_API_KEY from hardcoded values to ${{ secrets.* }} references, eliminating credential exposure.
• Replaced the old "Copy shieldci.yml config" and "Copy target repo to engine" steps with "Build Kali Docker image" and "Install test app dependencies" steps.

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