appveyor.yml: Fix build failures due to missing stdbuf

[Holzhaus]

Currently all appveyor builds fail with:

bash -c "stdbuf -oL -eL dist*/mixxx-test.exe --gtest_output=xml:test_results.xml 2>&1"
bash: stdbuf: command not found
Command exited with code 127

[daschuer]

Unfortunately the AppVeyor timed out.
Can you add a fake commit to trigger AppVeyor again?

Do you know what was the reason to use "stdbuf"?
What is the drawback without it?

[Holzhaus]

Restarted here: https://ci.appveyor.com/project/Holzhaus/mixxx/build/job/8dym0c6na1q3e0hg

[Holzhaus]

Do you know what was the reason to use "stdbuf"?
What is the drawback without it?

No. Maybe to get faster log output by working around the kernel's pipe buffer?

[Holzhaus]

Gosh, I times out every single time. Maybe we should switch the appveyor builds to CMake now. I'll check if it works with PR #2332.