Switch back to `ENV{PKG_CONFIG}` to ensure that
FindPkgConfig doesn't overwrite `PKG_CONFIG_EXECUTABLE`.
Since CMake 3.3 supports the environment variable,
it's safe to use that.
Switch back to `ENV{PKG_CONFIG}` to ensure that
FindPkgConfig doesn't overwrite `PKG_CONFIG_EXECUTABLE`.
Since CMake 3.3 supports the environment variable,
it's safe to use that.