Fix cmake error in pedantic mode

Suppresses error:
CMake Error (dev) at build/coverage/_deps/fmt-src/CMakeLists.txt:208 (set):
  uninitialized variable 'CMAKE_MODULE_PATH'
This error is for project developers. Use -Wno-error=dev to suppress it.

Author: rlalik

CMakeLists.txt

@@ -205,8 +205,7 @@ if (NOT CMAKE_RUNTIME_OUTPUT_DIRECTORY)
   set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/bin)
 endif ()
 
-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH}
-  "${CMAKE_CURRENT_SOURCE_DIR}/support/cmake")
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/support/cmake")
 
 include(CheckCXXCompilerFlag)
 include(JoinPaths)