Merged PR 739353: Silence innocuous warnings in rolling build

bxl.ps1 is used to run this test build in RunCheckinTests.cmd. But that test build doesn't have the mounts that bxl.ps1 expects. Silence the warning for that build specifically

Author: mpysson

RunCheckInTests.cmd

@@ -267,7 +267,7 @@ endlocal && exit /b 0
     set start=!time!
     set stepName=Running SymLink Tests
     call :StatusMessage !stepName!
-        call :RunBxl -Use RunCheckinTests %BUILDXL_ARGS% /c:%ENLISTMENTROOT%\Public\Src\Sandbox\Windows\DetoursTests\SymLink1\config.dsc /TraceInfo:RunCheckinTests=Symlink /logsDirectory:%~dp0out\Logs\SymLinkTest\ -SharedCacheMode disable
+        call :RunBxl -Use RunCheckinTests %BUILDXL_ARGS% /c:%ENLISTMENTROOT%\Public\Src\Sandbox\Windows\DetoursTests\SymLink1\config.dsc /TraceInfo:RunCheckinTests=Symlink /logsDirectory:%~dp0out\Logs\SymLinkTest\ -SharedCacheMode disable /nowarn:855
         rmdir /s /q %ENLISTMENTROOT%\Public\Src\Sandbox\Windows\DetoursTests\SymLink1\Out
         if !ERRORLEVEL! NEQ 0 (exit /b 1)
     call :RecordStep "!stepName!" !start!