av1_c_vs_simd_encode.sh: add --fresh to cmake call

This ensures a clean build directory is used for each build. This
addresses missing `-std=c++11` flags for libwebm files when running a
modified version of this script on macOS Arm.

Change-Id: I4b56ced98121a4ae2e07b6cd5320e8addb0e3cff

test/av1_c_vs_simd_encode.sh

diff --git a/test/av1_c_vs_simd_encode.sh b/test/av1_c_vs_simd_encode.sh
index a55db5e..4fc481a 100755
--- a/test/av1_c_vs_simd_encode.sh
+++ b/test/av1_c_vs_simd_encode.sh
@@ -299,7 +299,7 @@
   mkdir -p $tmp_build_dir
   cd $tmp_build_dir
 
-  local cmake_common_args="-DCONFIG_EXCLUDE_SIMD_MISMATCH=1 \
+  local cmake_common_args="--fresh -DCONFIG_EXCLUDE_SIMD_MISMATCH=1 \
            -DCMAKE_BUILD_TYPE=Release \
            -DENABLE_CCACHE=1 \
            '-DCMAKE_C_FLAGS_RELEASE=-O3 -g' \