Add .NET Core 3.1 SDK in .NET Isolated Core Tools image

[anthonychu]

Currently needed to build the .NET Isolated function apps properly

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• New Unit tests were added for the changes made and CI is passing.

[matt-psaltis]

Please pretty please.

[ankitkumarr]

/azp run

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[ankitkumarr]

/azp run "v3 dotnet build"

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