cmd/dist: log timing to $GOBUILDTIMELOGFILE

We can't make all.bash faster if we can't measure it.
Measure it.

Change-Id: Ia5da791d4cfbfa1fd9a8e905b3188f63819ade73
Reviewed-on: https://go-review.googlesource.com/73990


Reviewed-by: Ian Lance Taylor <iant@golang.org>