depends: Fix build with MULTIPROCESS=1 in Guix environment

In the Guix environment, `${BASEPREFIX}/${HOST}/native/bin` is added to
the `PATH` environment variable, causing CMake to search for package
configurations in the `native` subdirectory first.

Explicitly specifying the top-priority search prefixes for the
`Libmultiprocess` and `LibmultiprocessNative` packages resolves the
issue.

depends/toolchain.cmake.in

@@ -168,7 +168,8 @@ endif()
 
 if("@multiprocess@" STREQUAL "1")
   set(WITH_MULTIPROCESS ON CACHE BOOL "")
-  set(LibmultiprocessNative_DIR "${CMAKE_CURRENT_LIST_DIR}/native/lib/cmake/Libmultiprocess" CACHE PATH "")
+  set(Libmultiprocess_ROOT "${CMAKE_FIND_ROOT_PATH}" CACHE PATH "")
+  set(LibmultiprocessNative_ROOT "${CMAKE_FIND_ROOT_PATH}/native" CACHE PATH "")
 else()
   set(WITH_MULTIPROCESS OFF CACHE BOOL "")
 endif()