Revise flag from `automatic` to `manual` is irreversible

On Hackage, when revising a package metadata, revising a flag from `automatic` to `manual` is permitted, but the reverse is not. Hence, after a mistaken revision from `automatic` to `manual`, the package metadata gets stuck with the wrong setting.

Example: [hs-onnxruntime-capi-0.1.0.0](https://hackage-content.haskell.org/package/hs-onnxruntime-capi-0.1.0.0) is stuck with `pkg-config` as a manual flag, while it was an automatic flag before. (It _should_ be manual with a default set to false, but that's now an unreachable configuration.)