build-script: use a private sandbox for integration tests

This does not solve all issues, see https://bugs.swift.org/browse/SR-321

Author: gribozavr

utils/build-script-impl

@@ -2325,7 +2325,7 @@ if [[ "${INSTALLABLE_PACKAGE}" ]] ; then
     fi
     if [[ "${TEST_INSTALLABLE_PACKAGE}" ]] ; then
         PKG_TESTS_SOURCE_DIR="${WORKSPACE}/swift-integration-tests"
-        PKG_TESTS_SANDBOX_PARENT="/tmp/swift_package_sandbox"
+        PKG_TESTS_SANDBOX_PARENT="$(build_directory swift_package_sandbox none)"
 
         if [[ "$(uname -s)" == "Darwin" ]] ; then
             PKG_TESTS_SANDBOX="${PKG_TESTS_SANDBOX_PARENT}"/"${TOOLCHAIN_PREFIX}"