Add NuGet Caching to Azure Pipelines Build

IKVM adds more than 1 GB of NuGet dependencies and has a big impact on the length of a build. So, we should add some caching so the build server only needs to download NuGet packages if they change.

This was done on Azure Pipelines a long time ago

It was removed in e017bda153bb09e453b7bfee7f137c47386099d8
It was added in d484b330b38887d352473309ba14b2a9ff5ca5ae

However, the cache key wasn't set up to read all `*.props`, `*.targets`, and `*.csproj` files, so when we updated version numbers of packages, it broke the build on the server. Also, I couldn't figure out how to invalidate the cache once the key was corrected, so it was simpler just to remove the caching at the time. But now that it will have a more significant impact on performance, this should be added again.

The caching change should also be propagated to ICU4N, J2N, RandomizedTesting, Spatial4n, and Morfologik.Stemming build pipelines.