ci: Fix test reports not being uploaded

With 1c112bb7 (ci: Move test reports into temporary directory, 2022-01-13), we have moved our test reports into a temporary directory to prepare for running tests as an unprivileged user. This change broke our ability to upload these test reports as artifacts though because GitLab only supports artifacts which are relative to the build root.

Fix this issue by instead writing test reports into a new directory in our build root that's writeable by the unprivileged test user.