Remove BoxBackup install step from Windows unibuild

We don't need to install anything in C:\Program Files\BoxBackup, and often
don't have permission to do that anyway.

(cherry picked from commit 2d31a31)

infrastructure/cmake/windows/CMakeLists.txt

@@ -124,5 +124,6 @@ ExternalProject_Add(boxbackup
 		-DDEBUG=${DEBUG}
 		${boxbackup_cmake_args}
 		${SUB_CMAKE_EXTRA_ARGS}
-	STEP_TARGETS configure build install
+	INSTALL_COMMAND ${CMAKE_COMMAND} -E echo "No install step needed"
+	STEP_TARGETS configure build
 )