Fix #1161 - stdout is line-buffered during command execution

[joerick]

Fix #1161 - making stdout line-buffered.

I can't think how to add a unit test for this because capfd keeps stdout and stderr separately. But on my machine, this-

python -m test.test_projects test.test_0_basic.basic_project /tmp/basic_project
(cd /tmp/basic_project && CIBW_BEFORE_BUILD="python -uc 'import sys, time; print(1, file=sys.stdout); time.sleep(0.1); print(2, file=sys.stderr)'" cibuildwheel --platform linux)

Prints

2
1

before this PR and ...

1
2

after.

[henryiii]

There should be no harm, so maybe let's get this in before release?