allow override env-variables in load_catalog

[bdilday]

I found that I was unable to change a catalog URI after it had been configured by an environment variable. Example,

import os
os.environ["PYICEBERG_CATALOG__SOMEDB__URI"] = "https://service-one.io"
from pyiceberg.catalog import load_catalog
catalog = load_catalog("somedb", uri="https://service-two.io")

[tries to connect to https://service-one.io]

I think the more intuitive behavior is that the URI would be initialized based on the environment variable (if present) but that the argument passed to load_catalog would be able to override the value. This PR updates the behavior so that the passed in argument takes precedence, by swapping the lhs and rhs preference in the merge_config method.

[rdblue]

The proposed behavior sounds reasonable to me.

[Fokko]

Thanks @bdilday for raising this, and I agree that the explicitly passed-in configuration should take preference. Can you rebase against the latest main branch? The CI should turn green 👍

[Fokko]

Thanks for fixing this @bdilday