bug#43364: with-output-to-port works with file ports

Hi,

In an interesting (or perhaps maddening) inconsistency,
'with-output-to-port' captures stdout from system* here

(call-with-output-file "/tmp/test.log"
  (lambda (port)
    (with-output-to-port
      port
      (lambda ()
        (system* "mktemp" "-d")))))

but 'with-output-to-string' does not do so here

(with-output-to-string
  (lambda ()
    (system* "mktemp" "-d")))

According to lloda on #guile, system* handles the redirection only
when the current ports are file ports. Thanks for that pointer!

Kind regards
Felix