CI: push tests vs merge groups

[François Garillot]

Our current CI triggers on push to dev as well as merge groups:
https://github.com/lurk-lab/neptune/blob/9a6c931d158ebbfeb2a301f45d637642d65f0779/.github/workflows/check-downstream-compiles.yml
https://github.com/lurk-lab/neptune/blob/d8b4eeadd8acc9d9e8d9d510605c954f1410aa60/.github/workflows/rust.yml#L4-L9

• the push tests are for the most part redundant,
• the check-downstream-compiles job is meant only as a warning, and so is useless on merge_group or push.