Make UndockedRegFreeWinRT use same vcpkg_installed directory (microsoft#5454)

When setting the path for the vcpkg installed directory in the shared
props file, we were using `$(ProjectDir)\..\`. This works fine for most
projects because they are one level under the `src` directory, but
`UndockedRegFreeWinRT` is deeper in the tree so it would end up having
its own separata vcpkg_installed directory.
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Author: florelis

src/vcpkg.props

@@ -2,7 +2,7 @@
 <Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup Label="Vcpkg">
     <VcpkgEnableManifest>true</VcpkgEnableManifest>
-    <VcpkgInstalledDir>$(ProjectDir)..\vcpkg_installed</VcpkgInstalledDir>
+    <VcpkgInstalledDir>$(MSBuildThisFileDirectory)\vcpkg_installed</VcpkgInstalledDir>
   </PropertyGroup>
   <PropertyGroup Label="Vcpkg" Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
     <VcpkgTriplet>x64</VcpkgTriplet>