buildman: Correct counting of build failures on retry

When a build is to be performed, buildman checks to see if it has already
been done. In most cases it will not bother trying again. However, it was
not reading the return code from the 'done' file, so if the result was a
failure, it would not be counted. This depresses the 'failure' count stats
that buildman prints in this case.

Fix this bug by always reading the return code.

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