ImmutableCollections: include the key in the "duplicate key" exception message

This makes the exception message much more useful in log files.
Disable the NuGet cache when installing Microsoft.DotNet.BuildTools

Fix #206
Respond to code review feedback
Merge pull request #185 from akoeplinger/key-in-exception-message

ImmutableCollections: include the key in the "duplicate key" exception message
Merge pull request #210 from PatrickMcDonald/buildtools-cache

Disable the NuGet cache when installing Microsoft.DotNet.BuildTools