ci: Fix again build-previous-qemu

ci: Fix again build-previous-qemu

The build-previous-qemu job is now trying to fetch from the upstream
repository, but the tag is only fetched into FETCH_HEAD:

$ git remote add upstream https://gitlab.com/qemu-project/qemu 00:00
$ git fetch upstream $QEMU_PREV_VERSION 00:02
warning: redirecting to https://gitlab.com/qemu-project/qemu.git/
From https://gitlab.com/qemu-project/qemu
 * tag                     v8.2.0     -> FETCH_HEAD
$ git checkout $QEMU_PREV_VERSION 00:02
error: pathspec v8.2.0 did not match any file(s) known to git

Fix by fetching the tag into the checkout itself.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>