Werk #15656: Agent TLS registration: Fix `500 Internal Server Error` during register and status

When registering new agents (cmk-agent-ctl register) or querying the status of registered agents (cmk-agent-ctl status), users might have encountered the error

Error: Request failed with code 500 Internal Server Error: Internal Server Error (!!)

This happened if the path /opt/tmp existed on the monitoring server and the corresponding site user did not have read permissions for this path. If this happened, the log file var/log/agent-receiver/error.log contained errors such as

[ERROR] Exception in ASGI application
Traceback (most recent call last):
  ...
PermissionError: [Errno 13] Permission denied: '../../../tmp/check_mk/data_source_cache/push-agent/my-host'

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