Fix install.sh crash when piped from curl

[rorpage]

Summary

• BASH_SOURCE[0] is unbound when bash reads from stdin (the curl ... | sudo bash path), and set -euo pipefail made it an immediate fatal error
• Even if that were worked around, the script would then fail trying to install local unit files that don't exist on the target machine
• Fix guards BASH_SOURCE[0] with :- so SCRIPT_DIR defaults to "" when piped, then downloads unit files (game-over-man.service, game-over-man.timer) and env.example from GitHub when no local script directory is available

Mirrors the fix from rorpage/now-you-noaa#4.

Test plan

• curl -fsSL https://raw.githubusercontent.com/rorpage/game-over-man/main/deploy/systemd/install.sh | sudo bash no longer crashes on line 14
• sudo bash deploy/systemd/install.sh from a local clone still uses local unit files (no unnecessary downloads)
• Source-build fallback (REPO_ROOT logic) still works from a local clone when no release binary is found

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