Update Mixin Xref Build to use current build versions

MSBuild has been manually switched to 203 to prevent the following exception:

Failed tests summary: 1 (1 new)
(new) MixinXRef.UnitTests.dll: MixinXRef.UnitTests.InvolvedTypeFinderTest.FindInvolvedTypes_WithTargetClassInheritance details »

Stacktraces:
(new) MixinXRef.UnitTests.dll: MixinXRef.UnitTests.InvolvedTypeFinderTest.FindInvolvedTypes_WithTargetClassInheritance
Expected: equivalent to < <MixinXRef.UnitTests.TestDomain.UselessObject, isTarget: True, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.ClassInheritsFromUselessObject, isTarget: True, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.Mixin1, isTarget: False, isMixin: True, # of targets: 2>, <MixinXRef.UnitTests.TestDomain.Mixin4, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.BaseMixinWithInheritedMethod, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.MixinWithInheritedMethod, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.UnusedMixin, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.Explore.SimpleMemberOverrideTest+TemplateMixin, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.Explore.MemberOverrideWithInheritanceTest+CustomMixin, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.BaseMemberOverrideTestClass+Mixin1, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.ComposedInterfacesTestClass+MyMixin, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.HiddenMemberTestClass+Mixin1, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.MemberOverrideTestClass+Mixin1, isTarget: False, isMixin: False, # of targets: 0> >
But was: < <MixinXRef.UnitTests.Explore.MemberOverrideWithInheritanceTest+CustomMixin, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.Explore.SimpleMemberOverrideTest+TemplateMixin, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.BaseMemberOverrideTestClass+Mixin1, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.BaseMixinWithInheritedMethod, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.ClassInheritsFromUselessObject, isTarget: True, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.ComposedInterfacesTestClass+MyMixin, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.HiddenMemberTestClass+Mixin1, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.MemberOverrideTestClass+Mixin1, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.Mixin1, isTarget: False, isMixin: True, # of targets: 2>, <MixinXRef.UnitTests.TestDomain.Mixin4, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.MixinWithInheritedMethod, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.UnusedMixin, isTarget: False, isMixin: False, # of targets: 0>, <MixinXRef.UnitTests.TestDomain.UselessObject, isTarget: True, isMixin: False, # of targets: 0> >
at MixinXRef.UnitTests.InvolvedTypeFinderTest.FindInvolvedTypes_WithTargetClassInheritance() in C:\Build\47effa48bc3bc252\working\UnitTests\MixinXRef.UnitTests\InvolvedTypeFinderTest.cs:line 151

For Nightly Builds, the SDK Root cannot be found.