* Bump VSMac to 8.1.0.2742 to fix msbuild issues

This is required to get the support for the msbuild `ToolsVersion`
change from `15.0` to `Current`.

* [tests][msbuild] Fix Binding resources test with updated msbuild

Test failure with updated msbuild and vsmac 8.1:

```
Xamarin.iOS.Tasks.NativeReferencesNoEmbedding("iPhone").ShouldNotUnnecessarilyRebuildBindingProject(True)
     Binding project build did not create package?
  Expected: True
  But was:  False

at Xamarin.iOS.Tasks.NativeReferencesNoEmbedding.ShouldNotUnnecessarilyRebuildBindingProject (System.Boolean framework) [0x000a0] in <74b8f7d8a53e40109916d305bb4d7403>:0
at (wrapper managed-to-native) System.Reflection.RuntimeMethodInfo.InternalInvoke(System.Reflection.RuntimeMethodInfo,object,object[],System.Exception&)
at System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo cul
ture) [0x0006a] in <0519fa732e8845b6a809ce9180f541db>:0
```

The test builds the project multiple times. Before the 3rd build, the project
file's timestamp is updated and expects that the binding package will be
rebuilt. But it is not, because the target `_CreateBindingResourcePackage`
doesn't depend on that project file. So, add that to the target inputs.

* [nuget] Use xibuild to run nuget

Fix errors seen during `nuget restore` for tests:

```
Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/xammac_tests/xammac_tests.csproj(213,3): error MSB4024: The imported project file "/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Mac/Xamarin.Mac.CSharp.targets" could not be loaded. Could not find file "/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Mac/Xamarin.Mac.CSharp.targets"
```

* [xibuild] Fix incorrect mscorlib.dll being used

The `GuiUnit_NET_4_5` project, when built with `xibuild` uses the wrong `mscorlib.dll`.

From dotnet#5760 (comment) :

```
- mscorlib.dll is being used from mono/4.5 and the other system assemblies are from mono/4.5-api
- GuiNet* project is built with xibuild

What is happening here is:

	xibuild sets[1] `SetToolsetProperty ("TargetFrameworkRootPath", FrameworksDirectory + Path.DirectorySeparatorChar);`
	which points to `/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/xbuild-frameworks`.
	This causes $(FrameworkPathOverride) to be set[2] to `/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/xbuild-frameworks/.NETFramework/v4.5`,
	but that doesn't have a mscorlib.dll, so it gets reset[3] to /Library/Frameworks/Mono.framework/Versions/5.22.0/lib/mono/4.5/.

	If we don't set TargetFrameworkRoothPath, then we get `FrameworkPathOverride = /Library/Frameworks/Mono.framework/Versions/5.22.0/lib/mono/4.5-api`,
	causing `_ExplicitReference=/Library/Frameworks/Mono.framework/Versions/5.22.0/lib/mono/4.5-api/mscorlib.dll`(correct one) to be used.
```

Fixes dotnet#5760

1. https://github.com/xamarin/xamarin-macios/blob/master/tools/xibuild/Main.cs#L209
2. https://github.com/mono/msbuild/blob/xplat-master/src/Tasks/Microsoft.Common.CurrentVersion.targets#L79
3. https://github.com/mono/msbuild/blob/xplat-master/src/Tasks/Microsoft.Common.CurrentVersion.targets#L84

* Revert "Workaround dotnet#5760 in generator csproj"

This reverts commit 9bd927b.

The previous commit for xibuild removes the need for this.