Fix Windows CI: bunzip2 before tar, partial "git clean"

"tar" hangs on extracting from the bz2 archive. Workaround is to first unzip, then extract. Seems to be the same issue as reported here: https://github.com/actions/virtual-environments/issues/282

Exclude "build" folder from "git clean" because currently there is a blocked/busy file (workaround until someone reboots the Windows runner or kills those processes).

Additional notes: File attributes like described in gitlab-org/gitlab-runner#1839 or gitlab-org/gitlab-runner#3185 (closed) appear not to be an issue.