Improve NuGet package cache expiration

[runesoerensen]

Currently, the NuGet package layer cache will continue to grow (until the max restore limit is reached, since #58). This can be improved with more granular logic in a number of ways:

• Simply track the number of times the cache layer has been restored, and invalidate it after x number of times. implemented in Implement basic NuGet cache purging #58
• On related metadata changes (e.g. arch/os/sdk changes).
• Use this (undocumented) feature to determine when a package was last accessed, and purge packages that were not restored since the cache was restored.
• Wait for a better cache clean-up/expiration policy to be implemented in NuGet A better cache clean-up and expiration policy NuGet/Home#4980