github: add GHA security scanner workflow (zizmor)

[simondeziel]

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[simondeziel]

@tomponline would you mind enabling https://github.com/canonical/setup-lxd/security/quality, please?

[Copilot]

Pull request overview

Adds an automated GitHub Actions security scanning workflow (zizmor) and adjusts existing workflows/config to align with that security posture.

Changes:

• Add a new zizmor GitHub Actions workflow to analyze workflow security.
• Refactor composite action steps to pass inputs via step env variables (instead of direct expression interpolation).
• Remove the “Require GHA pinning” step from the test workflow and add a Dependabot update cooldown.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
action.yml	Routes inputs.* into step env variables used by bash scripts.
.github/workflows/zizmor.yml	Introduces zizmor-based workflow security analysis in CI.
.github/workflows/test.yml	Removes the workflow pinning enforcement step from tests.
.github/dependabot.yml	Adds a Dependabot cooldown setting for GitHub Actions updates.

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[tomponline]

@tomponline would you mind enabling https://github.com/canonical/setup-lxd/security/quality, please?

Done