bisect-builds.py: React to SIGINT while downloading
bisect-builds.py: React to SIGINT while downloading

Previously ^C was only handled after the download fully completed. This
was annoying, and with the bug where archived builds were almost 1 GB in
file size (crbug.com/560184), this problem became even worse.

Review URL: https://codereview.chromium.org/1592573004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370244}

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