exec does not close stream in node >= 14.0.0 <14.17.0

[parmentf]

The exec statement of @ezs/spawn uses the spawn event to resolve a promise.

The pool code assume that this event exists if node version is greater or equal to 14.0.0.

https://github.com/Inist-CNRS/ezs/blob/master/packages/spawn/src/pool.js#L92

But nodejs documentation says that the event was added at node 14.17.0 & 15.1.0

Consequence: when using a node between 14.0 and 14.17, the promise is not resolved (nor rejected), and the process is hanging.