Fix: Different startup steps implementation since other fix fails to restore (#622)

## Description

Pull Request #601 implementation fails to restore. I believe I narrowed
the problem down to the added `disabledPackageSources` property in
nuget.config, though my access to the Nuget feed isn't currently
working.

This solution takes the disabling effect out of nuget.config and puts it
in the Directory.Packages.props. In the .props file it conditionally
restores the private Nuget feed based on the single bool prop,
`AccessToNugetFeed`, defined in Directory.Build.props.

Credit: [GitHub issue 1](NuGet/Home#6045),
[GitHub issue 2](NuGet/Home#3972)

---------

Co-authored-by: Benjamin Michaelis <[email protected]>

Author: Joshua-Lester3

Directory.Build.props

@@ -7,10 +7,9 @@
     <PreserveCompilationContext>true</PreserveCompilationContext>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <DockerDefaultTargetOS>Linux</DockerDefaultTargetOS>
-    <AccessToNugetFeed>true</AccessToNugetFeed>
 
     <!-- https://aka.ms/vs-build-acceleration -->
     <AccelerateBuildsInVisualStudio>True</AccelerateBuildsInVisualStudio>
     <UserSecretsId>18e91e0d-ea2d-490f-b77e-ec008f9d09ec</UserSecretsId>
   </PropertyGroup>
-</Project>
+</Project>

Directory.Packages.props

@@ -3,6 +3,14 @@
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
     <CentralPackageTransitivePinningEnabled>false</CentralPackageTransitivePinningEnabled>
     <ToolingPackagesVersion>1.1.1.4415</ToolingPackagesVersion>
+    <AccessToNugetFeed>true</AccessToNugetFeed>
+    <RestoreSources>
+      https://api.nuget.org/v3/index.json;
+    </RestoreSources>
+    <RestoreSources Condition="$(AccessToNugetFeed)">
+      $(RestoreSources);
+      https://pkgs.dev.azure.com/intelliTect/_packaging/EssentialCSharp/nuget/v3/index.json;
+    </RestoreSources>
   </PropertyGroup>
   <ItemGroup Condition="$(AccessToNugetFeed)">
     <PackageVersion Include="ContentFeedNuget" Version="$(ToolingPackagesVersion)" />