FindPkgConfig: split args if loaded from environment

It's common for some people to use the PKG_CONFIG environment variable to not only load a custom pkg-config/pkgconf but also to load some default arguments such as --static or --keep-system-libs which often worked since shell scripts would call $PKG_CONFIG --libs pkg without quotes, but this breaks FindPkgConfig since it uses the full string as argv[0] and might try looking for a binary called pkgconf --static, instead of looking for pkgconf and adding --static as the argv[1]

Fixes: #22305 (closed)
Topic-rename: FindPkgConfig-PKG_CONFIG-args