run: reject JSON output in scope mode

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

Before, systemd-run printed JSON metadata to stdout and then executed
the scope command on the same stdout. The combined stream was not valid
JSON, so reject --json= in scope mode.

Follow-up for fe5a6c47af

src/run/run.c

@@ -705,6 +705,10 @@ static int parse_argv(int argc, char *argv[]) {
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                        "--no-block is not supported in --scope mode.");
 
+        if (arg_scope && sd_json_format_enabled(arg_json_format_flags))
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "--json= is not supported in --scope mode.");
+
         if (arg_stdio != ARG_STDIO_NONE) {
                 if (with_trigger || arg_scope)
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL),

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

@@ -120,6 +120,7 @@ systemd-run --scope \
             --property=RuntimeMaxSec=infinity \
             true
 (! systemd-run --scope --ignore-failure false)
+(! systemd-run --scope --json=short true)
 
 : "Transient scope (user daemon)"
 # FIXME: https://github.com/systemd/systemd/issues/27883