GH-39023: [C++][CMake] Add missing launcher path conversion for ExternalPackage (#45349)

### Rationale for this change

We need to convert it on Windows.

### What changes are included in this PR?

Convert it to CMake path.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #39023

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: kou

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -988,9 +988,11 @@ endif()
 
 # Enable s/ccache if set by parent.
 if(CMAKE_C_COMPILER_LAUNCHER AND CMAKE_CXX_COMPILER_LAUNCHER)
+  file(TO_CMAKE_PATH "${CMAKE_C_COMPILER_LAUNCHER}" EP_CMAKE_C_COMPILER_LAUNCHER)
+  file(TO_CMAKE_PATH "${CMAKE_CXX_COMPILER_LAUNCHER}" EP_CMAKE_CXX_COMPILER_LAUNCHER)
   list(APPEND EP_COMMON_CMAKE_ARGS
-       -DCMAKE_C_COMPILER_LAUNCHER=${CMAKE_C_COMPILER_LAUNCHER}
-       -DCMAKE_CXX_COMPILER_LAUNCHER=${CMAKE_CXX_COMPILER_LAUNCHER})
+       -DCMAKE_C_COMPILER_LAUNCHER=${EP_CMAKE_C_COMPILER_LAUNCHER}
+       -DCMAKE_CXX_COMPILER_LAUNCHER=${EP_CMAKE_CXX_COMPILER_LAUNCHER})
 endif()
 
 if(NOT ARROW_VERBOSE_THIRDPARTY_BUILD)