list_collections() output doesn't update when sia_url (or ssa_url) is changed

```python
from astroquery.ipac.irsa import Irsa
Irsa.sia_url = "https://irsadev.ipac.caltech.edu/simulated/SIA"
Irsa.list_collections(servicetype="SIA")
```
outputs a table with 111 rows from the default SIA endpoint ("https://irsadev.ipac.caltech.edu/SIA") instead of 4 rows present at the simulated endpoint. This is problematic because `Irsa.query_sia()` is correctly talking to the updated `sia_url` so I was expecting `list_collections()` to behave similarly to tell me which collection id/name I can use.

This probably is a duplicate since we talked about it at some point. If so, feel free to close it and cross-link existing issue.

Start by tracing Irsa.list_collections and compare how it selects the endpoint with Irsa.query_sia, using the example in the issue against the default and simulated SIA URLs. Done means changing sia_url or ssa_url makes list_collections query that endpoint and report its collections, including the four rows at the simulated endpoint.

Written by the indexing model from the issue text.