augment instead of replace the package source list when %PB_RestoreSource% is specified

build.cmd

@@ -572,7 +572,7 @@ if "%RestorePackages%" == "true" (
     %_ngenexe% install %_nugetexe%  /nologo
     set _nugetoptions=-PackagesDirectory packages -ConfigFile %_nugetconfig%
     if not "%PB_RESTORESOURCE%" == "" (
-        set _nugetoptions=!_nugetoptions! -Source %PB_RESTORESOURCE%
+        set _nugetoptions=!_nugetoptions! -FallbackSource %PB_RESTORESOURCE%
     )
 
     echo _nugetoptions=!_nugetoptions!

build.sh

@@ -450,7 +450,7 @@ if [ "${RestorePackages:-true}" = 'true' ]; then
 
     _nugetoptions="-PackagesDirectory packages -ConfigFile $_nugetconfig"
     if [ "$PB_RESTORESOURCE" != "" ]; then
-        _nugetoptions="$_nugetoptions -Source $PB_RESTORESOURCE"
+        _nugetoptions="$_nugetoptions -FallbackSource $PB_RESTORESOURCE"
     fi
 
     eval "$_nugetexe restore packages.config $_nugetoptions"