fix Nexus asset query issue. (#724)

* fix Nexus asset query issue.

was not constructing continuationToken URL correctly.

* ignore temp artifact files.

* unit test for download-artifact

* update cached download-artifact.js

* ignore download-artifact test from zap unit tests.

* download-artifact: add unit test / delegate to Github if specified branch is not cached on Nexus

intended for internal jenkins PRs that specifies a non-cached ZAP branch
on Nexus