Add BUILD_ALWAYS to ensure BearSource rebuilds

This change ensures that `BearSource` is rebuilt whenever its sources
are modified. The `BUILD_ALWAYS` option in CMake's `ExternalProject`
module enforces this behavior. For more details, see the CMake
documentation
https://cmake.org/cmake/help/latest/module/ExternalProject.html.

CMakeLists.txt

@@ -83,6 +83,8 @@ ExternalProject_Add(BearSource
             -DCMAKE_MODULE_LINKER_FLAGS:STRING=${CMAKE_MODULE_LINKER_FLAGS}
             -DROOT_INSTALL_PREFIX:PATH=${CMAKE_INSTALL_PREFIX}
             ${CMAKE_CACHE_ARGS_EXTRA}
+        BUILD_ALWAYS
+            1
         TEST_BEFORE_INSTALL
             1
         TEST_COMMAND