update_openapijson invoke task is not idempotent

[ml-evs]

Just realised this is what is causing it, after many confused commits where they get reordered again.

For some reason, the release deployment is reordering the "x-optimade-" fields in the OpenAPI schema, see: 5243791

I assume this is caused by some dependency version differences between the CI and local setups, since the CI diff only uses JSON diff to check differences (which is invariant to field ordering -- we could make it not so).

EDIT: I actually get the same behaviour locally after multiple re-runs of invoke update-openapijson. We should somehow fix the ordering here...