buildman: Use bytes for the environment

At present we sometimes see problems in gitlab where the environment has
0x80 characters or sequences which are not valid UTF-8.

Avoid this by using bytes for the environment, both internal to buildman
and when writing out the 'env' file. Add a test to make sure this works
as expected.

Reported-by: Marek Vasut <marex@denx.de>
Fixes: e5fc79ea ("buildman: Write the environment out to an 'env' file")
Signed-off-by: Simon Glass <sjg@chromium.org>