fix(job-runner): move job manifest path (#6517)

We found that we cannot rely on configmaps that mount as a specific file
in k8s: they don't update without a restart
(https://kubernetes.io/docs/concepts/storage/volumes/#using-subpath)

![Documentation about
subpath](https://github.com/user-attachments/assets/1f18b7f8-6b77-454b-b70a-d7efd5b797a3)

But, we cannot overwrite the contents of snuba/manual_jobs/ by mounting
configuration there because we rely on source code in that directory. So
we are moving this config to a new directory that doesn't pollute
anything else (`snuba/env`).

Next step:
- updating the mount in ops to point to snuba/env

snuba/manual_jobs/runner.py

@@ -41,7 +41,7 @@ def __init__(self, job_id: str, status: JobStatus):
 
 
 def _read_manifest_from_path(filename: str) -> Mapping[str, JobSpec]:
-    local_root = os.path.dirname(__file__)
+    local_root = os.path.join(os.path.dirname(__file__), "..", "env")
 
     with open(os.path.join(local_root, filename)) as stream:
         contents = simplejson.loads(stream.read())