Why does the Arcade SDK project enable GeneratePackageOnBuild?

[dagood]

arcade/src/Microsoft.DotNet.Arcade.Sdk/Microsoft.DotNet.Arcade.Sdk.csproj

Line 10 in c21a409

<GeneratePackageOnBuild>true</GeneratePackageOnBuild>

From #4030 (comment).

If this is not necessary, it should also be removed from other SDK projects, probably. If it is necessary, it should be commented.

/cc @alexperovich

[tmat]

IIRC @natemcmaster added it so that unit test project that tests the SDK can use the built package.