Make workflow run workflow in source repo context to prevent secrets …

[bj00rn]

Make workflow run workflow in source repo context to prevent secrets exposure.

• pull_request_target event will be run in target repo context and will have access to secrets
• pull_request_target event SHA is latests commit in target base branch, and running ui-tests here does not make sense as ui-tests will be run on latest commit in target branch.
• Remove file upload since workflow no longer has access to secrets.