minor: fix clippy in nightly.

[jackwener]

Which issue does this PR close?

N/A

Rationale for this change

What changes are included in this PR?

fix clippy in nightly.

Are these changes tested?

Are there any user-facing changes?

[jackwener]

BTW, there is more error in cargo clippy of nightly.

error[E0428]: the name `ALLOC` is defined multiple times
  --> benchmarks/src/bin/tpch.rs:62:1
   |
58 | static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;
   | ------------------------------------------------------------ previous definition of the value `ALLOC` here
...
62 | static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ALLOC` redefined here
   |
   = note: `ALLOC` must be defined only once in the value namespace of this module

error: cannot define multiple global allocators
  --> benchmarks/src/bin/tpch.rs:62:1
   |
58 | static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;
   | ------------------------------------------------------------ previous global allocator defined here
...
61 | #[global_allocator]
   | ------------------- in this procedural macro expansion
62 | static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot define a new global allocator
   |
   = note: this error originates in the attribute macro `global_allocator` (in Nightly builds, run with -Z macro-backtrace for more info)

[waynexia]

These changes look good to me.

BTW, there is more error in cargo clippy of nightly.

Looks like we need to add some feature gates for them? like

#[cfg(feature="snmalloc")]
static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;

#[cfg(feature="mimalloc")]
static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;

[ursabot]

Benchmark runs are scheduled for baseline = 3c1e4c0 and contender = f9f40bf. f9f40bf is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java