Fix make run-tests

`<SetEnvironmentVariable/>` was failing because `JI_JVM_PATH` was
being set to an empty string.

Author: jonpryor

build-tools/scripts/RunNUnitTests.targets

@@ -27,7 +27,7 @@
     <SetEnvironmentVariable Name="MONO_TRACE_LISTENER"   Value="Console.Out" />
     <SetEnvironmentVariable Name="JAVA_INTEROP_GREF_LOG" Value="bin\Test$(Configuration)\g-%(_TestAssembly.Filename).txt" />
     <SetEnvironmentVariable Name="JAVA_INTEROP_LREF_LOG" Value="bin\Test$(Configuration)\l-%(_TestAssembly.Filename).txt" />
-    <SetEnvironmentVariable Name="JI_JVM_PATH"           Value="$(JdkJvmPath)" />
+    <SetEnvironmentVariable Name="JI_JVM_PATH"           Value="$(JdkJvm11Path)" />
     <Exec
         Command="$(_NUnit) $(NUNIT_EXTRA) %(_TestAssembly.Identity) $(_Run) --result=&quot;TestResult-%(Filename).xml&quot; --output=&quot;bin\Test$(Configuration)\TestOutput-%(Filename).txt&quot;"
         WorkingDirectory="$(_TopDir)"