Avoid apt install for preinstalled gh

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Problem

The action runs apt-get update && apt-get install gh on every job. GitHub-hosted runners already include the GitHub CLI, so an intermittent package mirror stall can prevent the security scan from starting.

Change

Use the runner's existing gh binary and fail immediately with a clear error if a self-hosted runner does not provide it.

Validation

• Confirmed the check and version output work with the installed gh binary.
• git diff --check passed.