Try suggested fix

Author: pitrou

ci/scripts/cpp_build.sh

@@ -34,7 +34,13 @@ if [ ! -z "${CONDA_PREFIX}" ] && [ "${ARROW_EMSCRIPTEN:-OFF}" = "OFF" ]; then
   echo -e "===\n=== Conda environment for build\n==="
   conda list
 
-  export ARROW_CMAKE_ARGS="${ARROW_CMAKE_ARGS} -DCMAKE_AR=${AR} -DCMAKE_RANLIB=${RANLIB}"
+  if [ -n "${AR}" ]; then
+    ARROW_CMAKE_ARGS+=" -DCMAKE_AR=${AR}"
+  fi
+  if [ -n "${RANLIB}" ]; then
+    ARROW_CMAKE_ARGS+=" -DCMAKE_RANLIB=${RANLIB}"
+  fi
+  export ARROW_CMAKE_ARGS
   export ARROW_GANDIVA_PC_CXX_FLAGS=$(echo | ${CXX} -E -Wp,-v -xc++ - 2>&1 | grep '^ ' | awk '{print "-isystem;" substr($1, 1)}' | tr '\n' ';')
 elif [ -x "$(command -v xcrun)" ]; then
   export ARROW_GANDIVA_PC_CXX_FLAGS="-isysroot;$(xcrun --show-sdk-path)"