Fix code scanning warning

Author: tspascoal

src/OctoshiftCLI.Tests/Octoshift/Services/ArchiveUploadersTests.cs

@@ -73,7 +73,7 @@ public void Constructor_Should_Use_Default_When_Environment_Variable_Not_Set()
 
         environmentVariableProviderMock
             .Setup(x => x.GithubOwnedStorageMultipartBytes(false))
-            .Returns((string)null);
+            .Returns(() => null);
 
         // Act
         var archiveUploader = new ArchiveUploader(githubClientMock.Object, logMock.Object, retryPolicy, environmentVariableProviderMock.Object);