Release CI fails when resource doesn't have "upstream source"

In script-exporter (vm charm), we [have](https://github.com/canonical/script-exporter-operator/blob/6fb096df0c6b123259bcab7b206018c53c70fd27/charmcraft.yaml#L31) a file resource,

```yaml
resources:
script-exporter-binary:
type: file
description: Binary for Script Exporter
filename: script_exporter
```

so CI [keeps](https://github.com/canonical/script-exporter-operator/actions/workflows/release.yaml) [failing](https://github.com/canonical/script-exporter-operator/actions/runs/15436891616/job/43445841977) with `KeyError: 'upstream-source'`.

Start with charmcraft.yaml at the linked resources entry and the release.yaml workflow, then reproduce the failing release job to inspect where the missing upstream-source key is assumed. Trace the resource metadata used by CI and make the release workflow handle this file resource correctly; done means the release workflow passes for script-exporter.

Written by the indexing model from the issue text.