SPIKE: running propose-downstream in action

[lbarcziova]

After migrating building SRPMs for Copr builds in Copr, the sandcastle will be used only for propose-downstream execution. However, we could get rid of sandcastle fully if we utilised the GH actions instead.

The expectation here would be that we would trigger a GH action that would run the propose downstream, wait for its completion (see) and report the state/do additional handling.

The outcome of this card should be the requirements for implementation of this and possible problems so that we can decide whether we want to do this.

This came up when discussing #1662

[lachmanfrantisek]

Still valid, we still want to get rid of sandcastle.