run: reject JSON output with stdio forwarding

Reproducer:
  sudo systemd-run --wait --pipe --json=short /bin/echo hi

Before, systemd-run wrote its JSON metadata to stdout and then passed
the command stdout through the same stream. The combined output was not
valid machine-readable JSON, so reject the conflicting modes.

Follow-up for fe5a6c47af

test/units/TEST-74-AUX-UTILS.run.sh

@@ -256,6 +256,7 @@ systemd-run --wait --pty true
 systemd-run --wait --machine=.host --pty true
 systemd-run --json=short true | jq . >/dev/null
 systemd-run --json=pretty true | jq . >/dev/null
+(! systemd-run --wait --pipe --json=short true)
 (! SHELL=/bin/false systemd-run --quiet --shell)
 
 (! systemd-run)