ci: Produce snap artifact every build, not just on manual runs

[ashuntu]

The workflow that builds the Snap only actually uploads the artifact on manual runs. It would be useful to produce an artifact every build to aid in testing ability, especially for those who don't want to checkout the code themselves.

This PR removes the workflow_dispatch condition of the artifact upload so that it should produce an artifact every time it runs.

You can even see the artifact produced for this PR with this change!: https://github.com/ubuntu/app-center/actions/runs/7890140289?pr=1599

[tim-hm]

@ashuntu I think there's a type-o in your commit message: Product -> Produce.

[d-loose]

I think we've been using the workflow_dispatch trigger on all our snap workflows in order not to constantly produce unnecessary artifacts, as there are organization-wide limits (although I don't know what they are). It might be wise to reduce the artifact retention period for the repo.

[tim-hm]

Good pt @d-loose. I suggest dev always builds on new commits ... and feature branches only on manual invocation?