program: fix computation of app path with single file

When publishing GCM as a 'single file' application, the computed path to
the entry executable is no longer correct.

On .NET Core 3.1, using Assembly.Location resolves to the temporary
extracted DLL file path. On .NET 5 Assembly.Location always returns the
empty string.

Since .NET 5, published single-file apps no longer use the self-
extraction model, and are real single file with all assemblies and
native libraries statically linked/bundled.

We now use the Environment.GetCommandLineArgs() method to get the raw
underlying "argv" arguments, which argv[0] is the absolute file path to
the entry executable.

src/shared/Git-Credential-Manager/Program.cs

@@ -3,6 +3,7 @@
 using System;
 using System.IO;
 using System.Reflection;
+using System.Runtime.InteropServices;
 using Atlassian.Bitbucket;
 using GitHub;
 using Microsoft.AzureRepos;
@@ -36,16 +37,17 @@ public static void Main(string[] args)
 
         private static string GetApplicationPath()
         {
-            Assembly entryAssembly = Assembly.GetExecutingAssembly();
-            if (entryAssembly is null)
-            {
-                throw new InvalidOperationException();
-            }
+            // Assembly::Location always returns an empty string if the application was published as a single file
+            bool isSingleFile = string.IsNullOrEmpty(Assembly.GetEntryAssembly()?.Location);
 
-            string candidatePath = entryAssembly.Location;
+            // Use "argv[0]" to get the full path to the entry executable - this is consistent across
+            // .NET Framework and .NET >= 5 when published as a single file.
+            string[] args = Environment.GetCommandLineArgs();
+            string candidatePath = args[0];
 
-            // Strip the .dll from assembly name on Mac and Linux
-            if (!PlatformUtils.IsWindows() && Path.HasExtension(candidatePath))
+            // If we have not been published as a single file on .NET 5 then we must strip the ".dll" file extension
+            // to get the default AppHost/SuperHost name.
+            if (!isSingleFile && Path.HasExtension(candidatePath))
             {
                 return Path.ChangeExtension(candidatePath, null);
             }