exec-invoke: fix wrong errno in log_error_errno for setenv failure

When setenv("CREDENTIALS_DIRECTORY") fails, log_error_errno() was
called with the stale return value of exec_context_get_credential_directory()
(which is >= 0 on success) instead of errno.

The %m in the format string correctly expands from libc's errno, so
the human-readable log message was unaffected. However, the structured
journal field ERRNO= received an incorrect value (0 or positive),
making automated log analysis and alerting on this failure unreliable.

Signed-off-by: dongshengyuan <dongshengyuan@uniontech.com>

src/core/exec-invoke.c

@@ -1090,7 +1090,7 @@ static int ask_password_conv(
 
                                 if (creds_dir) {
                                         if (setenv("CREDENTIALS_DIRECTORY", creds_dir, /* overwrite= */ true) < 0)
-                                                return log_error_errno(r, "Failed to set $CREDENTIALS_DIRECTORY: %m");
+                                                return log_error_errno(errno, "Failed to set $CREDENTIALS_DIRECTORY: %m");
                                 } else
                                         (void) unsetenv("CREDENTIALS_DIRECTORY");