run: reject JSON output for trigger units

Reproducer:
  unit=run-json-trigger-$(date +%s)
  sudo systemd-run --json=short --unit="$unit" \
      --on-active=30s \
      /bin/true

Before, trigger mode accepted --json=short but printed only human-readable
"Running timer as unit" and "Will run service" lines. Reject the option
until trigger mode has structured output.

Follow-up for fe5a6c47af

src/run/run.c

@@ -735,6 +735,10 @@ static int parse_argv(int argc, char *argv[]) {
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                        "--timer-property= has no effect without any other timer options.");
 
+        if (sd_json_format_enabled(arg_json_format_flags) && with_trigger)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "--json= is not compatible with path, socket or timer operations.");
+
         if (arg_wait) {
                 if (arg_no_block)
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL),

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

@@ -257,6 +257,7 @@ 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)
+(! systemd-run --json=short --on-active=30s true)
 (! SHELL=/bin/false systemd-run --quiet --shell)
 
 (! systemd-run)