dashboard: increase TIMEOUT_SCALE to 2 for linux-amd64-longtest
dashboard: increase TIMEOUT_SCALE to 2 for linux-amd64-longtest

runtime tests take about 180s in long mode, too close to the timeout
limit (180 seconds), and this is causing sporadic timeout failures
like

  https://build.golang.org/log/cda568a39f396e3fee64778a475528be452632ed

Bump up the builder's timeout by 2x.

Fixes golang/go#25629

Change-Id: I77e0f86f0b236cb953457ebdcbfc5763c2c6d54b
Reviewed-on: https://go-review.googlesource.com/115016
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build