[Java.Interop] Fix BuildJniEnvironment_g_cs target outputs.

Commit fbc70cd changed where `jni.c` was generated, but:

 1. Didn't update the //Target/@outputs for `jni.c`
 2. Didn't create $(IntermediateOutputPath).

(2) resulted in a build failure on Jenkins:

	System.IO.DirectoryNotFoundException:
	Could not find a part of the path ".../java-interop/src/Java.Interop/obj/Debug//jni.c"

Fix the BuildJniEnvironment_g_cs target.

Author: jonpryor

src/Java.Interop/Java.Interop.csproj

@@ -151,8 +151,8 @@
   <PropertyGroup>
     <Runtime Condition="'$(OS)' != 'Windows_NT'">mono</Runtime>
   </PropertyGroup>
-  <Target Name="BuildJniEnvironment_g_cs" Inputs="$(JNIEnvGenPath)\jnienv-gen.exe" Outputs="Java.Interop\JniEnvironment.g.cs;$(OutputPath)\jni.c">
-    <MakeDir Directories="$(OutputPath)" />
+  <Target Name="BuildJniEnvironment_g_cs" Inputs="$(JNIEnvGenPath)\jnienv-gen.exe" Outputs="Java.Interop\JniEnvironment.g.cs;$(IntermediateOutputPath)\jni.c">
+    <MakeDir Directories="$(IntermediateOutputPath)" />
     <Exec Command="$(Runtime) &quot;$(JNIEnvGenPath)\jnienv-gen.exe&quot; Java.Interop\JniEnvironment.g.cs $(IntermediateOutputPath)\jni.c" />
   </Target>
   <ItemGroup>