cmd/gitops-pusher: support alternate api-server URLs

Fixes https://github.com/tailscale/coral/issues/90

Signed-off-by: Denton Gentry <dgentry@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