Fix run_desktop_tests.sh script

The script begins with `set -eux` and therefore will fail when using an
undefined variable. This is the case with `TRAVIS_OS_NAME` outside of
Travis.

run_desktop_tests.sh

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-set -eux
+set -ex
 
 echo "Checking formatting..."
 cargo fmt --all -- --check