buildman: Add an encoding to the out-env file

buildman: Add an encoding to the out-env file

The environment may contain some unicode characters. At least that is what
seemed to happen on one commit:

Building current source for 1 boards (0 threads, 64 jobs per thread)
    0    0    0 /1       -1      (starting)
Traceback (most recent call last):
  File ".../tools/buildman/buildman", line 64, in <module>
    ret_code = control.DoBuildman(options, args)
  File "tools/buildman/control.py", line 372, in DoBuildman
    options.keep_outputs, options.verbose)
  File ".../tools/buildman/builder.py", line 1704, in BuildBoards
    results = self._single_builder.RunJob(job)
  File ".../tools/buildman/builderthread.py", line 526, in RunJob
    self._WriteResult(result, job.keep_outputs, job.work_in_output)
  File ".../tools//buildman/builderthread.py", line 349, in _WriteResult
    print('%s="%s"' % (var, env[var]), file=fd)
UnicodeEncodeError: 'ascii' codec can't encode characters in position
	311-312: ordinal not in range(128)

The problem defies repetition with any change at all to buildman. But
let's set an encoding in any case.

Signed-off-by: Simon Glass <sjg@chromium.org>