fix: k8s container volume mounts as list (#1868)

As described in the body of PR
#1858, there's a typo whereby
the property `container.volume_mounts` is overwritten, rather than
appended to.

### QC

* [x] The PR contains a test case for the changes or the changes are
already covered by an existing test case. (this is allegedly tested as
part of the k8s tests).
* [ ] ~The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).~

snakemake/executors/__init__.py

@@ -1853,10 +1853,8 @@ def run(self, job, callback=None, submit_callback=None, error_callback=None):
         container.args = ["-c", exec_job]
         container.working_dir = "/workdir"
         container.volume_mounts = [
-            kubernetes.client.V1VolumeMount(name="workdir", mount_path="/workdir")
-        ]
-        container.volume_mounts = [
-            kubernetes.client.V1VolumeMount(name="source", mount_path="/source")
+            kubernetes.client.V1VolumeMount(name="workdir", mount_path="/workdir"),
+            kubernetes.client.V1VolumeMount(name="source", mount_path="/source"),
         ]
 
         node_selector = {}