[infra] Follow 302s when fetching swarming_bot.zip.
[infra] Follow 302s when fetching swarming_bot.zip.

This seems necessary to obtain the file, but the redirect appears
obfuscated in some way, so we can't just go to it directly, it must be
part of the flow.

Bug: skia:9438
Change-Id: I79599274ecc8f743a397171b8b907747e3861fb6
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/280897
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Auto-Submit: Weston Tracey <e7e27cb74e46abd7b999223aa4f27901ab7e58c6@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot