With GOBUILDTIMELOGFILE set, make.bash logs the starting time using

  $ echo $(date) > file

and expects to be able to read the date back with

  time.Parse(time.UnixDate)

but in some locales the default date format is not the same as
time.UnixDate; for example on LC_TIME="en_GB.UTF-8"

  $ locale date_fmt
  %a %e %b %H:%M:%S %Z %Y

Fix this by setting LC_TIME=C before the date command invocation.

Fixes #22541

Change-Id: I59bf944bb868e2acdd816c7e35134780cdbfc6a6
Reviewed-on: https://go-review.googlesource.com/75370
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>