[build] Change $(JavacClasspathSeparator) to $(JavaPathSeparator) (#1373)

In `build-tools/Java.Interop.Sdk/Sdk/Sdk.props`, the property
`$(JavaPathSeparator)` is defined, but the condition for the
non-Windows case references an undeclared property
`$(JavacClasspathSeparator)`.

It causes on Windows, javac -classpath with multiple jars fails.

This is likely a typo. This change corrects it to
`$(JavaPathSeparator)` to match the property being defined.

Author: Mliybs

build-tools/Java.Interop.Sdk/Sdk/Sdk.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
     <JavaPathSeparator Condition=" $([MSBuild]::IsOSPlatform('windows')) ">;</JavaPathSeparator>
-    <JavaPathSeparator Condition=" '$(JavacClasspathSeparator)' == '' ">:</JavaPathSeparator>
+    <JavaPathSeparator Condition=" '$(JavaPathSeparator)' == '' ">:</JavaPathSeparator>
   </PropertyGroup>
 </Project>