Fix custom-acceptance-tests artifacts capture.
Fix custom-acceptance-tests artifacts capture.

The test config[1] sets the artifact directory to /tmp/artifacts, which
didn't match where this was putting them. They therefore weren't being
found up by the upload task.

This change was missed in 7325556 when we switched away from using the
simulated errand environment.

[1] manifests/cf-manifest/manifest/900-cf-tests.yml line 23

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf