x/build: scaleway networking is super slow

[bradfitz]

Our Scaleway instances are taking tens of minutes to download 7 MB:

# docker logs 75f0795b2529
2017/11/15 20:43:58 network is up.
2017/11/15 20:43:58 Downloading https://storage.googleapis.com/go-builder-data/buildlet.linux-arm to ./buildlet.exe ...
root@scw-e8738f:~# 
root@scw-e8738f:~# 
root@scw-e8738f:~# date
Wed Nov 15 20:52:57 UTC 2017

# curl -O https://storage.googleapis.com/go-builder-data/buildlet.linux-arm
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  2 7010k    2  179k    0     0  17298      0  0:06:55  0:00:10  0:06:45 18419^C

It's painfully slow.

Maybe we need to cache harder and mirror the buildlet in Paris or in their object store.

But that doesn't help if we have to push 40 MB of source at each build.

This bug is about investigating where Scaleway networking is slow.

In the meantime we need to disable our linux-arm trybots.

[gopherbot]

Change https://golang.org/cl/78032 mentions this issue: dashboard: remove linux-arm as a trybot

[gopherbot]

Change https://golang.org/cl/325694 mentions this issue: dashboard, cmd/coordinator: remove all Scaleway builders