Add PB_IsStable (StabilizePackageVersion alias)

RoslynTools-using repos look for PB_IsStable.

Author: dagood

repos/dir.props

@@ -53,6 +53,7 @@
     <EnvironmentVariables Include="DotNetCoreSdkDir=$(DotNetCliToolDir)" />
     <EnvironmentVariables Include="DotNetBuildToolsDir=$(ProjectDir)Tools/" />
     <EnvironmentVariables Include="StabilizePackageVersion=$(UseStableVersions)" Condition="'$(UseStableVersions)' != ''" />
+    <EnvironmentVariables Include="PB_IsStable=$(UseStableVersions)" Condition="'$(UseStableVersions)' != ''" />
     <EnvironmentVariables Include="PackageVersionStamp=$(VersionStamp)" Condition="'$(VersionStamp)' != ''" />
 
     <!-- Turn off node reuse for source build because repos use conflicting versions