shared: log error when execve fail

If there is an error with the execv call in fork_agent the
program exits without any meaningful log message. Log the
command and errno so the user gets more information about
the failure.

Fixes: #33418

Signed-off-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>

src/shared/exec-util.c

@@ -606,5 +606,6 @@ int fork_agent(const char *name, const int except[], size_t n_except, pid_t *ret
         va_end(ap);
 
         execv(path, l);
+        log_error_errno(errno, "Failed to execute %s: %m", path);
         _exit(EXIT_FAILURE);
 }