remote-store: don't log raw stderr by default

For remote stores the log messages are already forwarded as structured
STDERR_RESULT messages so the old format is duplicate information.  But
still included with -vvv since it could be useful for debugging
problems.

    $ nix build -L /nix/store/nl71b2niws857ffiaggyrkjwgx9jjzc0-foo.drv --store ssh-ng://localhost
    Hello World!
    foo> Hello World!
    [1/0/1 built] building foo

Fixes #3556

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