C#: Don't initialise DependabotProxy on Windows or macOS

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/DependabotProxy.cs

@@ -27,6 +27,13 @@
 
         internal static DependabotProxy? GetDependabotProxy(ILogger logger, TemporaryDirectory tempWorkingDirectory)
         {
+            // Setting HTTP(S)_PROXY and SSL_CERT_FILE have no effect on Windows or macOS,
+            // but we would still end up using the Dependabot proxy to check for feed reachability.
+            // This would result in us discovering that the feeds are reachable, but `dotnet` would
+            // fail to connect to them. To prevent this from happening, we do not initialise an
+            // instance of `DependabotProxy` on those platforms.
+            if (SystemBuildActions.Instance.IsWindows() || SystemBuildActions.Instance.IsMacOs()) return null;
+
             // Obtain and store the address of the Dependabot proxy, if available.
             var host = Environment.GetEnvironmentVariable(EnvironmentVariableNames.ProxyHost);
             var port = Environment.GetEnvironmentVariable(EnvironmentVariableNames.ProxyPort);
@@ -58,7 +65,7 @@
 
                 logger.LogInfo($"Stored Dependabot proxy certificate at {result.CertificatePath}");
 
                 result.Certificate = new X509Certificate2(result.CertificatePath);
             }
 
             return result;