Only run install-debian.wheezy.sysroot.py once during runhooks
This script was previously run three times, once for
each arch, with two of the three always being no-ops.
Instead just run the script once and have the script
figure out which image to download.
Second attempt at landing this. First attempt caused
failures on windows builders:
https://codereview.chromium.org/
834133003
TEST=trybots + 'gclient runhooks' locally
Review URL: https://codereview.chromium.org/
843643003
Cr-Commit-Position: refs/heads/master@{#310753}