Add missing parquet-variant-compute crate to CI jobs

[alamb]

Which issue does this PR close?

• Related to [EPIC] [Parquet] Implement Variant type support in Parquet #6736

Rationale for this change

I noticed in #7956 that some Clippy errors were introduced but not caught by CI.

What changes are included in this PR?

Add parquet-variant-compute to the CI for parqet-variant related PRs

Are these changes tested?

It is only tests

Are there any user-facing changes?

No

[alamb]

Aha -- the CI test caught a real error (fixed in #7956 (comment))

• https://github.com/apache/arrow-rs/actions/runs/16374023184/job/46269326079?pr=7963

    Checking arrow v55.2.0 (/__w/arrow-rs/arrow-rs/arrow)
    Checking parquet-variant-compute v0.1.0 (/__w/arrow-rs/arrow-rs/parquet-variant-compute)
error: this expression creates a reference which is immediately dereferenced by the compiler
  --> parquet-variant-compute/src/variant_array.rs:98:70
   |
98 |         let Some(metadata_field) = VariantArray::find_metadata_field(&inner) else {
   |                                                                      ^^^^^^ help: change this to: `inner`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
   = note: `-D clippy::needless-borrow` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::needless_borrow)]`

error: this expression creates a reference which is immediately dereferenced by the compiler
   --> parquet-variant-compute/src/variant_array.rs:109:64
    |
109 |         let Some(value_field) = VariantArray::find_value_field(&inner) else {
    |                                                                ^^^^^^ help: change this to: `inner`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

error: could not compile `parquet-variant-compute` (lib) due to 2 previous errors

[alamb]

In order to keep the CI clean, I am going to merge this without review