Extend user agent of publishing workflow

# Why?
We are currently setting a generic user agent here:
https://github.com/grafana/plugin-ci-workflows/blob/42327814c4814eef3a866e41626b48fc75568496/actions/plugins/publish/publish/publish.sh#L81

This could be extended to give us more visibility into who is actually calling the API as in - which repository or plugin it actually is.

Here is what it looks like in the gcom logs https://ops.grafana-ops.net/goto/df600zl8suu4gd?orgId=stacks-27821

```
"userAgent": "github-actions-shared-workflows:/plugins/publish",
```

# What / How
(whatever is simpler and more stable):
Change it either to:

```
"userAgent": "github-actions-shared-workflows:grafana//plugins/publish",
```
or (we seem to [have plugin.id in the same script](https://github.com/grafana/plugin-ci-workflows/blob/42327814c4814eef3a866e41626b48fc75568496/actions/plugins/publish/publish/publish.sh#L158))

```
"userAgent": "github-actions-shared-workflows://plugins/publish",
```