VERBOSE will be set by TRAVIS (but can also be set by user to diagnose slow
builds). Using an env var so that it automatically propagates to all the
sub-scripts and tools without having to pass it through tools/build explicitly.
gofmt no longer supports `tabs` and `tabwidth`, but we still require
exactly-4-space tabs to preserve the narrow layout on gobyexample.com,
so re-implement this functionality with sed.
