Update snapd to 2.39.1
Changelog:
- tests/unit/spread-shellcheck: temporary workaround for SC2251
- interfaces: special-case "snapd" in sanitizeSlotReservedForOS* helper
- cmd/snap: mangle descriptions that have indent > terminal width
- cmd/snap-confine: unshare per-user mount ns once
- tests: avoid adding spaces to the base snaps names
- systemd: workaround systemctl show quirks on older systemd versions