fix ci action to upload the artifact when acceptance tests fail

[lorenzo-cavazzi]

When using the actions/upload-artifact@v1 action in a step to upload the artifacts after failing the acceptance test, the action seem to complete (i.e. the files in the Artifacts section seem to be updated correctly) but it's still marked as failed with the following error:

Access to the path '/home/runner/work/renku-ui/renku-ui/test-artifacts/2020_10_30_1007_15/.gitattributes' is denied.
Exit code 1 returned from process: file name '/home/runner/runners/2.276.1/bin/Runner.PluginHost', arguments 'action "GitHub.Runner.Plugins.Artifact.PublishArtifact, Runner.Plugins"'.
Process completed with exit code 1.

An example is the following:

• code: https://github.com/SwissDataScienceCenter/renku-ui/blob/d718e740f2cd76ea13894f7eefee7207efc53209/.github/workflows/acceptance-tests.yml#L98-L109
• run: https://github.com/SwissDataScienceCenter/renku-ui/actions/runs/539906195