Fix engine_pyo3/build.rs missing clippy. (#12196)
Fix engine_pyo3/build.rs missing clippy. (#12196)

Previously this was causing errors on unrelated rust changes:
```
$ git commit --amend
* Running
   Compiling proc-macro2 v1.0.24
   ...
    Checking process_executor v0.0.1 (/home/jsirois/dev/pantsbuild/jsirois-pants/src/rust/engine/process_executor)
    Finished dev [unoptimized + debuginfo] target(s) in 2m 02s
* Checking formatting of Rust files
Ensuring generated code is present for downstream formatting checks...
    Checking tokio-stream v0.1.5
    Checking hashing v0.0.1 (/home/jsirois/dev/pantsbuild/jsirois-pants/src/rust/engine/hashing)
   Compiling bazel_protos v0.0.1 (/home/jsirois/dev/pantsbuild/jsirois-pants/src/rust/engine/process_execution/bazel_protos)
    Checking tower v0.4.6
    Checking tonic v0.4.1
    Finished dev [unoptimized + debuginfo] target(s) in 6.17s
* Checking Rust target headers
     Ignored package  is already installed, use --force to override
Rust targets didn't have correct prefix:
/home/jsirois/dev/pantsbuild/jsirois-pants/src/rust/engine/engine_pyo3/build.rs
```
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