Make source-build-reference-packages use a local package cache dir

[dagood]

The source-build-reference-packages build involves restoring some locally-built packages that are forced to be ref-only, unlike the copies available on the NuGet gallery. This may pollute the user's cache, or the user's cache might interfere with the SBRP build.

We should be careful not to affect any global/user state during the build.

We might just need to add something like export NUGET_PACKAGES="$scriptroot/packages/"?

[crummel]

[Scrub] This remains an issue. I built SBRP locally and the user NuGet cache was populated.