For PRs whose commits end up as HEAD of master like
bcc5c141bf,
there might be workflow runs associated with both PRs and pushes.
Only the push event is the one that will run fully and should be used to compare against.
So far it didn't distinguish between the two events, causing it to
sometimes pick the wrong one and then fail to download non-existent
artifacts: https://github.com/NixOS/nixpkgs/actions/runs/12092053414/job/33721377179
This commit fixes that by ensuring that the push event result is
compared against
- `env.mergedSha` is empty, so it checked out the master version by
default
- The process step used `needs.attrs.outputs.mergedSha`, but apparently
that's empty unless `attrs` is declared as a `needs`, even though
`outputs` implicitly depends on `attrs`
