Require switch on unofficial build

[ferventcoder]

The default one in the repository is for building source. If built in release mode, require a special switch to run --allow-unofficial-build or something like that.

When choco.exe detects that it is not using the official
publickeytoken, throw an error requiring an explicit override (this is
already overridden in debug builds). When AllowUnofficialBuild (--allow-unofficial-build) flag is
set to true, log a very important warning so that folks can receive
clues that the state of their system may be compromised.

[gep13]

Very cool! 👍

[usepowershell]

👍 👍 Yay for security and yay for being able to build without the "official" key for testing purposes (without having to do a bunch of modifications that make pull requests suck).