run: refuse --no-block when combined with --scope

In the systemd-run --scope mode, --no-block has no actual effect and will
be silently ignored. Therefore, this combination is explicitly rejected to
reduce confusion for users when using it.

Fixes: #42806

src/run/run.c

@@ -694,6 +694,10 @@ static int parse_argv(int argc, char *argv[]) {
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                        "--remain-after-exit and --service-type= are not supported in --scope mode.");
 
+        if (arg_scope && arg_no_block)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "--no-block is not supported in --scope mode.");
+
         if (arg_stdio != ARG_STDIO_NONE) {
                 if (with_trigger || arg_scope)
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL),