consolidate all nuget packages in a single folder

[licanhua]

Comment From @JunielKatarn on #3620: I believe we are placing several packages folders in the repo.
One in the root for the Universal and Desktop solution, and one for each Playground/Sample solution location.
If we use environment/build variables smartly, we should be able to consolidate all packages in a single folder, avoiding such duplication.

[licanhua]

Disk is not a issue on pipeline after these PRs even if we will use windows-2019 build machines :
#3773 reduce build flavors for RNWUniversalPR
#3768 Redirect build directory to C: on vs2017-win2016 build machine
#3771 Delete .pch after build on pipeline
#3734 Reduce build time on pipeline
#3729 Use ReactNative.V8JSI.Windows.0.1.6 and ReactNative.Hermes.Windows.0.1.6

In the future, if we hit disk space issue:

1. Apply Delete .pch after build on pipeline #3771 to the concerned project which is not in vnext folder
2. Spin another job to do the build other than build multiple solutions in one job. and it will save build time, also not hit the disk space issue