test: Use explicit ContentLength when uploading to S3
test: Use explicit ContentLength when uploading to S3

The build log was being uploaded with no content. I didn't track down
the exact reason, but adding this fixed it.

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

flynn/flynn,josephglanville/flynn,philiplb/flynn,rikur/flynn,lmars/flynn,lmars/flynn,rikur/flynn,lmars/flynn,benbjohnson/flynn,benbjohnson/flynn,philiplb/flynn,benbjohnson/flynn,josephglanville/flynn,lmars/flynn,flynn/flynn,benbjohnson/flynn,flynn/flynn,rikur/flynn,benbjohnson/flynn,josephglanville/flynn,josephglanville/flynn,josephglanville/flynn,flynn/flynn,rikur/flynn,flynn/flynn,rikur/flynn,josephglanville/flynn,lmars/flynn,philiplb/flynn,benbjohnson/flynn,philiplb/flynn,rikur/flynn,philiplb/flynn