Use a nuget.config file instead of specifying multiple sources on the command line

This works around dotnet/sdk#27202
It also has the benefit of using settings _only_ from the specified config file, ignoring the global nuget.config where package source mapping could interfere with the local source.

Author: 0xced

test/Serilog.Settings.Configuration.Tests/Support/TestApp.cs

@@ -101,14 +101,13 @@ async Task PackAsync()
 
     async Task RestoreAsync()
     {
-        var packagesDirectory = _workingDirectory.SubDirectory("packages");
+        // Can't use "--source . --source https://api.nuget.org/v3/index.json" because of https://github.com/dotnet/sdk/issues/27202 => a nuget.config file is used instead.
+        // It also has the benefit of using settings _only_ from the specified config file, ignoring the global nuget.config where package source mapping could interfere with the local source.
         var restoreArgs = new[] {
             "restore",
-            "--packages", packagesDirectory.FullName,
-            "--source", ".",
-            "--source", "https://api.nuget.org/v3/index.json",
+            "--configfile", "nuget.config",
             "-p:Configuration=Release",
-            $"-p:TargetFramework={TargetFramework}"
+            $"-p:TargetFramework={TargetFramework}",
         };
         await RunDotnetAsync(_workingDirectory, restoreArgs);
     }

test/TestApp/nuget.config

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <config>
+    <add key="globalPackagesFolder" value="packages" />
+  </config>
+  <packageSources>
+    <clear />
+    <add key="local" value="." />
+    <add key="nuget" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+</configuration>