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]`

Additionally adds RunCMake.FindPkgConfig ARGN test case

Fixes: #22305
Signed-off-by: Christopher Degawa <ccom@randomderp.com>