fix: unit test for object_storage

[chenkovsky]

Which issue does this PR close?

Rationale for this change

I have aws s3 configured in my develop env. These two tests fail.

cargo test object_storage::tests::s3_object_store_builder_resolves_region_when_none_provided
cargo test object_storage::tests::s3_object_store_builder_default

thread 'object_storage::tests::s3_object_store_builder_resolves_region_when_none_provided' panicked at datafusion-cli/src/object_storage.rs:746:9:
assertion `left == right` failed
  left: Some("xxxx")
 right: Some("eu-central-1")

What changes are included in this PR?

Set env to a non-existent file to avoid reading the default configuration file

Are these changes tested?

UT

Are there any user-facing changes?

No

[alamb]

Thank you @chenkovsky