[build] fix env vars causing gradle build error (#705)

Context: dotnet/android#5044 (comment)
Context: https://docs.microsoft.com/visualstudio/msbuild/exec-task

An env var can break the build such as running the following powershell:

	> $env:FOO="`nFoo.cs(123,1) error: oh dear!"

A new line in the string is required to hit an issue.  (```n`` is
PowerShell-ese for a `\n` newline.)

This results in a confusing build error:

	(_BuildJava target) ->
	    EXEC : Foo.cs(123,1) error : oh dear!

`gradlew -d` prints every environment variable, and if there is a line
that looks like an MSBuild error then `msbuild` will emit the error.
We can set `IgnoreStandardErrorWarningFormat="true"` to disable this.

This could commonly happen on CI systems, if a commit message contains
an error message.

In the case of Azure DevOps, `%BUILD_SOURCEVERSIONMESSAGE%` will
contain the full commit message.

I also fixed the `_CleanJava` target that was not running at all.
`java-source-utils.csproj` now properly cleans now.

tools/java-source-utils/Directory.Build.targets

@@ -11,6 +11,7 @@
         Command=""$(GradleWPath)" -d $(GradleArgs) -PjavaSourceVer=$(JavacSourceVersion) -PjavaTargetVer=$(JavacTargetVersion) jar"
         EnvironmentVariables="JAVA_HOME=$(JavaSdkDirectory);APP_HOME=$(GradleHome)"
         WorkingDirectory="$(MSBuildThisFileDirectory)"
+        IgnoreStandardErrorWarningFormat="true"
     />
     <Copy
         SourceFiles="build\libs\java-source-utils.jar"
@@ -72,7 +73,7 @@
     />
   </Target>
 
-  <Target Name="_CleanJava" DependsOnTargets="Clean">
+  <Target Name="_CleanJava" BeforeTargets="Clean">
     <Delete Files="$(OutputPath)java-source-utils.jar" />
     <Exec
         Command="&quot;$(GradleWPath)&quot; clean $(GradleArgs)"