Executable name requests were being treated as explicit requests to
install into system environments, but I don't think it should be as it's
implicit what environment you'll end up in. Following #4308, we allow
multiple executables to be found so we can filter here.
Concretely, this means `--system` is required to install into a system
environment discovered with e.g. `--python=python`. The flag is still
not required for cases where we're not mutating environment.