Resolved KeyError when fetching artifacts from ab.

while run_ab_key env variable is not set.

Test: fetch --type=ab --branch=git_oc-release --target=aosp_arm64_ab-userdebug --artifact_name=aosp_arm64_ab-img-{build_id}.zip --build_id=latest
Bug: 71780531
Change-Id: Ie84eea29c2b3b8a947f03f00af0e825324f6e36a