message: fix testing if a variable exists
diff: @@ -26,7 +26,7 @@ echo "Test host architecture"
  run_tests
  echo
  -if [ -n "$FOREIGN_TARGET" ]; then
  +if [ -n "${FOREIGN_TARGET+exists}" ]; then
  echo "Test foreign architecture ($FOREIGN_TARGET)"
  MIRI_TEST_TARGET="$FOREIGN_TARGET" run_tests
  echo
  