Enable changing artifact staging location

[chcosta]

This change adds an environment variable (ARCADE_STAGINGDIRECTORY) which can be used to control where artifacts and packages are populated.

This change is desired because there are some systems which run and needlessly scan artifacts simply because they reside in the sources folder. Those scan should be intentional. This change will allow us to avoid scanning unnecessary files which will dramatically reduce overall build times (times have not yet been measured but I believe this to be true).

This is the first of two changes which will be required to support this. Arcade provides a lot of templates which have hard-coded paths to the artifacts location, those will need to be updated after this change is published.

The following outlines the behavior changes on Windows. Behavior changes are the same for Linux/Mac.

Previous behavior

With -ci option:

• artifacts -> [repo root]\artifacts
• toolset -> [repo root]\artifacts\toolset
• .packages -> [repo root]\.packages (controllable via NUGET_PACKAGES env variable)

Without -ci option:

• artifacts -> [repo root]\artifacts
• toolset -> [repo root]\artifacts\toolset
• .packages -> [UserProfile]\.nuget\packages (controllable via NUGET_PACKAGES env variable)

New behavior

If ARCADE_STAGINGDIRECTORY is not set, behavior is unchanged.

If ARCADE_STAGINGDIRECTORY is set to a local path then:

With -ci option:

• artifacts -> [ARCADE_STAGINGDIRECTORY]\artifacts
• toolset -> [repo root]\.toolset (toolset moves out of artifacts folder because it requires discovering the repo's NuGet.config file
• .packages -> [ARCADE_STAGINGDIRECTORY]\.packages (controllable via NUGET_PACKAGES env variable)

Without -ci option:

• artifacts -> [ARCADE_STAGINGDIRECTORY]\artifacts
• toolset -> [repo root]\.toolset (toolset moves out of artifacts folder because it requires discovering the repo's NuGet.config file
• .packages -> [UserProfile]\.nuget\packages (controllable via NUGET_PACKAGES env variable)

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

Validated locally on Windows and Ubuntu with Arcade version 10.0.0-beta.25074.2 from the general-testing public feed.

[chcosta]

It's been a while since I made any change of this nature in Arcade so I've tagged anyone I can think of that might have an opinion on this type of change.

This change is to address dotnet/dnceng#2646