[mlir] Fix test failure with invalid piping syntax on old bash

Using '|&' syntax for piping both stdout and stderr is not supported by older bash. macOS pre-installs 3.2.57 as of today, and it causes test failure due to unexpected token '&'. We can use '2>&1' instead to make it compatible as much as possible.

```
******************** TEST 'MLIR :: mlir-cpu-runner/invalid.mlir' FAILED ********************
Script:
--
: 'RUN: at line 1';   not mlir-cpu-runner --no-implicit-module llvm-project/mlir/test/mlir-cpu-runner/invalid.mlir |& llvm-project/build/bin/FileCheck llvm-project/mlir/test/mlir-cpu-runner/invalid.mlir
--
Exit Code: 2

Command Output (stderr):
--
mlir-cpu-runner/Output/invalid.mlir.script: line 1: syntax error near unexpected token `&'
/mlir-cpu-runner/Output/invalid.mlir.script: line 1: `set -o pipefail;{ : 'RUN: at line 1';   not mlir-cpu-runner --no-implicit-module llvm-project/mlir/test/mlir-cpu-runner/invalid.mlir |& llvm-project/build/bin/FileCheck llvm-project/mlir/test/mlir-cpu-runner/invalid.mlir; }'
```

Reference: https://stackoverflow.com/questions/35384999/what-does-mean-in-bash

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D135347