Native toolset bootstrapper should support private cache

[chcosta]

The bootstrapper uses a cache under %userprofile%.netcoreeng. This is analogous to NuGet cache; but just like “-ci” uses a private cache ($(RepoRoot).packages or something like that), the native tool bootstrapper should be able to use a private cache for CI builds as well (for e.g., $(RepoRoot).netcoreeng).

• NuGet provides a handy away to clear its cache (nuget locals all -clear), but there is no handy analog for clearing the native-bootstrapper cache

[jonfortescue]

wait do dollar signs cause weird markdown rendering? $test$

[jonfortescue]

Whoa that's wild. is this intended for like math or something?

Anyway, we're not installing native tools anymore so this can be closed.