title: the configuration file 'secrets.json' was not found and is not optional (.NET 6)
body: <p>User Secrets error is being generated in a CI/CD pipeline, when secrets.json file shouldn't be expected.</p>
  <p>Steps:</p>
  <ol>
  <li>Create .NET 5 project</li>
  <li>Added user secrets.</li>
  <li>Code runs locally and in CI/CD pipelines.</li>
  <li>Upgrade to .NET 6 project (and preview NuGet 6.* packages)</li>
  </ol>
  <p>Code runs locally, but fails in CI/CD pipelines, with error:</p>
  <blockquote>
  <p>&quot;The configuration file 'secrets.json' was not found and is not optional.&quot;</p>
  </blockquote>
  <p>Expected:</p>
  <p>Code runs without the secrets.json file being present
  Configuration
  .NET 6, Microsoft.Extensions.Configuration.UserSecrets: 6.0.0-preview.1.21102.12</p>
  <p>Regression?
  This works in .NET 5, Microsoft.Extensions.Configuration.UserSecrets: 5.0.0.*</p>
  <pre><code>System.IO.FileNotFoundException: The configuration file 'secrets.json' was not found and is not optional. The physical path is '/home/runner/work/UserSecretsRegression/UserSecretsRegression/UserSecrets/UserSecrets.Tests/bin/Release/net6.0/secrets.json'.
    Stack Trace:
        at Microsoft.Extensions.Configuration.FileConfigurationProvider.HandleException(ExceptionDispatchInfo info)
     at Microsoft.Extensions.Configuration.FileConfigurationProvider.Load(Boolean reload)
     at Microsoft.Extensions.Configuration.FileConfigurationProvider.Load()
     at Microsoft.Extensions.Configuration.ConfigurationRoot..ctor(IList`1 providers)
     at Microsoft.Extensions.Configuration.ConfigurationBuilder.Build()
     at UserSecrets.Tests.UnitTest1.TestMethod1() in /home/runner/work/UserSecretsRegression/UserSecretsRegression/UserSecrets/UserSecrets.Tests/UnitTest1.cs:line 13
  </code></pre>