Make dotnet-nightly workaround version-agnostic

If the `Debug/dnxcore50` path does not exist, don't worry about it.

build.sh

@@ -35,7 +35,7 @@ mkdir -p $BIN/Modules
     cd src/Microsoft.PowerShell.Linux.Host
     dotnet publish --framework dnxcore50 --output $BIN
     # Temporary fix for dotnet publish
-    cp $BIN/Debug/dnxcore50/* $BIN
+    [[ -d $BIN/Debug/dnxcore50 ]] && cp $BIN/Debug/dnxcore50/* $BIN
     # Copy files that dotnet-publish does not currently deploy
     cp *_profile.ps1 $BIN
 )