ci: Harden CI, pin action versions, add zizmor linting.

[philipaconrad]

🔩 What code changed, and why?

This PR improves the security posture of our Github Actions workflows, in the same style as open-policy-agent/opa#8356 did for OPA proper. We now pin all actions, don't keep Git tokens around longer than necessary, and should now be immune to cache poisoning attacks during release builds.

👟 How to test

• Does CI still work?
• Do release builds still work? (Will be tested on next release)

⛓️ Related Resources

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