RHCOS CI for COSA fails on cosa buildextend-aliyun

Bug Report

See recent PRs CI failures:

• https://storage.googleapis.com/origin-ci-test/pr-logs/pull/coreos_coreos-assembler/2972/pull-ci-coreos-coreos-assembler-main-rhcos/1546600586357510144/build-log.txt

+ cosa_buildextend_all
+ cosa buildextend-aliyun
[INFO]: symlink is for a build and publish command
[INFO]: ore target aliyun found via symlink
[INFO]: Targeting architecture: x86_64
[INFO]: Targeting build: 412.86.202207112123-0
[INFO]: Processed build for: OpenShift 4 (RHCOS-x86_64) 412.86.202207112123-0
[INFO]: operating on rhcos-412.86.202207112123-0-aliyun.x86_64.qcow2
[INFO]: Processing the build artifacts
[INFO]: Staging temp image: /tmp/tmp.RL6gyT4H9s/tmp/tmp6173yce4/rhcos-412.86.202207112123-0-aliyun.x86_64.qcow2
Traceback (most recent call last):
  File "/usr/lib/coreos-assembler/cmd-buildextend-aliyun", line 115, in <module>
    build.ensure_built()
  File "/usr/lib/coreos-assembler/cosalib/build.py", line 263, in ensure_built
    self.build_artifacts()
  File "/usr/lib/coreos-assembler/cosalib/build.py", line 414, in build_artifacts
    self._build_artifacts(*args, **kwargs)
  File "/usr/lib/coreos-assembler/cosalib/qemuvariants.py", line 335, in _build_artifacts
    meta_patch = self.mutate_image()
  File "/usr/lib/coreos-assembler/cosalib/qemuvariants.py", line 251, in mutate_image
    self.set_platform()
  File "/usr/lib/coreos-assembler/cosalib/qemuvariants.py", line 226, in set_platform
    self.image_qemu, self.tmp_image, self.platform])
  File "/usr/lib/coreos-assembler/cosalib/qemuvariants.py", line 198, in image_qemu
    qimage = os.path.join(
  File "/usr/lib64/python3.10/posixpath.py", line 90, in join
    genericpath._check_arg_types('join', a, *p)
  File "/usr/lib64/python3.10/genericpath.py", line 152, in _check_arg_types
    raise TypeError(f'{funcname}() argument must be str, bytes, or '
TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'
{"component":"entrypoint","error":"wrapped process failed: exit status 1","file":"k8s.io/test-infra/prow/entrypoint/run.go:80","func":"k8s.io/test-infra/prow/entrypoint.Options.Run","level":"error","msg":"Error executing test process","severity":"error","time":"2022-07-11T21:40:05Z"} 
INFO[2022-07-11T21:40:12Z] Ran for 38m31s                               
ERRO[2022-07-11T21:40:12Z] Some steps failed:                           
ERRO[2022-07-11T21:40:12Z] 
  * could not run steps: step rhcos failed: test "rhcos" failed: the pod ci-op-rzr4i54k/rhcos failed after 20m44s (failed containers: test): ContainerFailed one or more containers exited

This is due to https://github.com/openshift/os/pull/853.

The failure reaches cosalib/qemuvariants.py, especially image_qemu at line 198, from cmd-buildextend-aliyun. Start by comparing this path with the openshift/os PR 853 change, then run the RHCOS CI buildextend-aliyun step and confirm it no longer raises the NoneType join error.