cmd/go: set GIT_TRACE_CURL for tests on builders

cmd/go: set GIT_TRACE_CURL for tests on builders

We have noticed a pattern of connection timeouts connecting to
github.com on the builders. Adding tracing may shed some light on the
underlying cause.

For #52545.

Change-Id: Ic73b494be9a3ea8ce3c7631ee8f62bd3d0d8e7bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/409575
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>