Don't load MSBuild.Framework in GitLoaderContext

Unit tests were failing because they load MSBuild assemblies from the
test output directory. Normally they're not available there, so
GitLoaderContext doesn't load them. But in tests, they were there, and
being loaded in both ALC.Default (from the main part of the test) and
a GitLoaderContext created a type mismatch

```
MissingMethodException: Method not found: 'Microsoft.Build.Framework.ITaskItem[] GitVersion.MSBuildTask.Tasks.UpdateAssemblyInfo.get_CompileFiles()'.
   at GitVersion.MSBuildTask.GitVersionTaskExecutor.UpdateAssemblyInfo(UpdateAssemblyInfo task)
   at GitVersion.MSBuildTask.GitVersionTasks.<>c__DisplayClass1_0.<UpdateAssemblyInfo>b__0(IGitVersionTaskExecutor executor) in D:\a\1\s\src\GitVersionTask\GitVersionTasks.cs:line 15
   at GitVersion.MSBuildTask.GitVersionTasks.ExecuteGitVersionTask[T](T task, Action`1 action) in D:\a\1\s\src\GitVersionTask\GitVersionTasks.cs:line 30
```

That was happening because the loaded `UpdateAssemblyInfo.CompileFiles`
was of type (`Microsoft.Build.Framework.ITaskItem[]` in the
`GitLoaderContext`) while it was looking for
(`Microsoft.Build.Framework.ITaskItem[]` in
`AssemblyLoadContext.Default`).

src/GitVersionTask.MsBuild/LibGit2Sharp/GitLoaderContext.cs

@@ -19,12 +19,21 @@ public sealed class GitLoaderContext : AssemblyLoadContext
 
         protected override Assembly Load(AssemblyName assemblyName)
         {
-            if (assemblyName.Name == entryPointAssembly.GetName().Name)
+            string simpleName = assemblyName.Name;
+
+            if (simpleName == entryPointAssembly.GetName().Name)
             {
                 return entryPointAssembly;
             }
 
-            var path = Path.Combine(Path.GetDirectoryName(typeof(GitLoaderContext).Assembly.Location), assemblyName.Name + ".dll");
+            if (simpleName == "Microsoft.Build.Framework")
+            {
+                // Delegate loading MSBuild types up to an ALC that should already have them
+                // once we've gotten this far
+                return null;
+            }
+
+            var path = Path.Combine(Path.GetDirectoryName(typeof(GitLoaderContext).Assembly.Location), simpleName + ".dll");
 
             if (File.Exists(path))
             {