app: fix installation dir calculation

On Windows, when invoked as a partial name (e.g., `manager`) our command
line argv[0] is just a file name, not a full file path! We incorrectly
assumed that `Environment.GetCommandLineArgs()[0]` was always absolute!

We should instead use `AppContext.BaseDirectory` that, in all
circumstances tested, all on TFMs and OSs, all publish options, and as a
.NET tool... returns the expected full, absolute path to the
installation directory.

Author: mjcheetham

src/shared/Core/ApplicationBase.cs

@@ -91,7 +91,7 @@ public static string GetEntryApplicationPath()
 
         public static string GetInstallationDirectory()
         {
-            return Path.GetDirectoryName(Environment.GetCommandLineArgs()[0]);
+            return AppContext.BaseDirectory;
         }
 
         /// <summary>