fix: use PathBuf to join paths

Summary

• fix: use PathBuf to join paths

this fixes the following bug:

picodata run --data-dir tmp/i1/
C> invalid socket path: tmp/i1//admin.sock
[supervisor:214288] no ipc message from child
[supervisor:214288] subprocess 214289 exited with code 1

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• Close #634 (closed)
• This is an attempt to backport !1035 (merged)
• Docs follow-up: not necessary