Generate random system_id for run-single mode

What does this MR do?

Generate a new random system_id for every gitlab-runner run-single invocation, preventing a <nil> dereference panic.

Alternative approaches

• Leave the system_id field blank, null or unspecified: discarded because it would require more code changes and might go against the purpose of #29390 (closed).
• Try to load an existing system_id from the file system: discarded because the system_id storage approach of !3758 (merged) uses paths relative to config.toml but this file is irrelevant for the gitlab-runner run-single command.

Why was this MR needed?

Closes #29592 (closed) and fixes the gitlab-runner run-single command.

What's the best way to test this MR?

See #29592 (closed) for a minimal example, albeit it's a bit worrying to realize that automated testing doesn't cover this functionality.

What are the relevant issue numbers?

#29592 (closed) & #29390 (closed)