CI: move logs/ to GITHUB_WORKSPACE

Artifacts can only be uploaded from inside the GITHUB_WORKSPACE. While
the Linux CI jobs run inside that per default, a special case-sensitive
mount outside the GITHUB_WORKSPACE is used for macOS builds.

To make log artifacts work for both macOS and Linux, move logs/ folder
to GITHUB_WORKSPACE on failures.

Signed-off-by: Paul Spooren <mail@aparcar.org>