nixos/jitsi-meet: fix systemd protection flag warning
Fixes systemd warning:
> /etc/systemd/system/jitsi-excalidraw.service:26: Failed to parse ProtectProc=true, ignoring: Invalid argument
when Jitsi-Meet Excalidraw Backend is enabled. `ProtectProc=true` is not a thing according to man pages, `ProtectProc=noaccess` is the most restrictive setting and likely what was originally intentioned.