Remote assets not re-downloaded on the server

[tacerus]

Issue Description

In a package with

<services>
  <service name="download_assets"/>
</services>

and a respective #!RemoteAssetUrl in the build description, the initially downloaded file seems to be preserved forever - using osc service rr or the "Trigger Services" button in the web interface causes the _service:download_assets:xxx file to temporarily disappear, but then reappear again with the same contents and change date as before.

Expected Result

Triggering the server side service should cause the remote asset to be freshly downloaded if it changed on the source.

How to Reproduce

1. Create a package with this template: https://build.opensuse.org/package/show/home:crameleon:test16/test-assets.
2. Write new data to the file behind RemoteAssetUrl.
3. Run osc service rr.
4. Observe the new data to not appear.

In the example package above, I played this through - meanwhile the content "v2" is served by the web server, but OBS still has the file with the content "v1" even after running services.

A hack to force a new download is to add a query parameter ignored by the remote web server.

Note that running the service locally appears to work fine.

[hennevogel]

@M0ses some caching that does not expire?