cmake_package_config_generate: Fix CMake warnings with CMake 3.23

Correct syntax to run CMake built-in command-line tools is `cmake -E`:
https://cmake.org/cmake/help/v3.23/manual/cmake.1.html#run-a-command-line-tool

This test was mistakenly using `cmake env ... cmake ...`, where `env` and second `cmake`
were interpreted as paths (of source directory or build directory), resulting
in "Ignoring extra path from command line" CMake warnings with CMake 3.23:
https://gitlab.kitware.com/cmake/cmake/-/commit/eacf1f879b0933509efbd4fb4d6d72ce99412aa7

cmake/TestPackageConfig.cmake

@@ -23,7 +23,7 @@ execute_process (
   # modified again (e.g., for MinGW AppVeyor CI builds) by adding back the
   # directory containing git.exe. Incidently, the Git installation directory
   # also contains sh.exe which causes MinGW Makefile generation to fail.
-  COMMAND ${CMAKE_COMMAND} env PATH=${PATH}
+  COMMAND ${CMAKE_COMMAND} -E env PATH=${PATH}
   ${CMAKE_COMMAND} -C ${INITIAL_CACHE}
     -G ${GENERATOR}
     ${_ADDITIONAL_ARGS}