ci: add Zizmor workflow

[tonyandrewmeyer]

This PR adds static workflow analysis using Zizmor. The standard Charm Tech configuration is used.

In addition, some minor findings are addressed:

• Default to empty permissions
• Pin actions
• Don't persist Git credentials

Two drive-by fixes to get CI to pass:

• Skip a test that needs an expired key. This is in apt, which has been migrated to charmlibs, so it seems harmless to skip.
• Modernise the integration tests for juju-systemd-notices. This avoids fixing issues in linked workflow actions.

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[james-garner-canonical]

Changes look reasonable to me. Nice to get broken tests working. We should probably look at archiving the repo soon, but better to archive it in a cleaner state than otherwise.