.github/workflows: remove 'ci skip' boilerplate

We've never used the "[ci skip]" magic commit header in our history,
across all our repos. This seems to be boilerplate we imported years
ago and have since been copying around our CI configs.

Signed-off-by: David Anderson <danderson@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f