Use env bultin in accelerate-matmul.mlir (#6991)

platform-independent env variable setting

Author: anmyachev

test/TritonGPU/accelerate-matmul.mlir

@@ -1,5 +1,5 @@
 // RUN: triton-opt %s -split-input-file --tritongpu-accelerate-matmul -verify-diagnostics=only-expected | FileCheck %s
-// RUN: TRITON_PREFER_TMEM_16x256_LAYOUT=1 triton-opt %s -split-input-file --tritongpu-accelerate-matmul | FileCheck %s --check-prefix=LAYOUT_16x256
+// RUN: env TRITON_PREFER_TMEM_16x256_LAYOUT=1 triton-opt %s -split-input-file --tritongpu-accelerate-matmul | FileCheck %s --check-prefix=LAYOUT_16x256
 
 // CHECK: #[[MMA:.+]] = #ttg.nvidia_mma<{versionMajor = 3, versionMinor = 0, warpsPerCTA = [4, 1], instrShape = [16, 16, 16]}>
 // CHECK: #[[MMA1:.+]] = #ttg.nvidia_mma<{versionMajor = 3, versionMinor = 0, warpsPerCTA = [4, 1], instrShape = [16, 64, 16]}>