Use SFML_ROOT to find SFML installation

This is line with modern CMake convention where -D<PackageName>_ROOT
is used to locate a given package's installation.

https://cmake.org/cmake/help/latest/variable/PackageName_ROOT.html#variable:%3CPackageName%3E_ROOT

tools/nuget/build.win.ps1

@@ -141,7 +141,7 @@ New-Item -ItemType Directory lib > $null
 $CSFMLLibDir = (Get-Item lib).FullName; # The directory where the final CSFML dlls are located
 
 cmake `
-    "-DSFML_DIR=$SFMLBuiltDir" `
+    "-DSFML_ROOT=$SFMLBuiltDir" `
     '-DCSFML_LINK_SFML_STATICALLY=1' `
     "-DCMAKE_LIBRARY_PATH=$SFMLExtLibs" `
     `