Minor: fix clippy in parquet-variant after logical conflict

[alamb]

Which issue does this PR close?

We generally require a GitHub issue to be filed for all bug fixes and enhancements and this helps us generate change logs for our releases. You can link an issue to this PR using the GitHub syntax.

• Follow on to [Variant] Improve getter API for VariantList and VariantObject #7757

Rationale for this change

Clippy is failing after merging #7757 due to a logical conflict and a new clippy release:

https://github.com/apache/arrow-rs/actions/runs/15924256410/job/44917809525

error: variables can be used directly in the `format!` string
   --> parquet-variant/src/variant/list.rs:130:25
    |
130 |             Err(err) => panic!("validation error: {}", err),
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
    = note: `-D clippy::uninlined-format-args` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::uninlined_format_args)]`
help: change this to
    |
1[30](https://github.com/apache/arrow-rs/actions/runs/15924256410/job/44917809525#step:6:31) -             Err(err) => panic!("validation error: {}", err),
130 +             Err(err) => panic!("validation error: {err}"),
    |

What changes are included in this PR?

Fix clippy to get CI clean on main

Are these changes tested?

By CI

Are there any user-facing changes?

No