acpid: Skip (rather than fail) if /proc/acpi doesn't exist

E.g. EC2 instances don't have ACPI.

modules/services/hardware/acpid.nix

@@ -105,6 +105,8 @@ in
         daemonType = "fork";
 
         exec = "acpid --confdir ${acpiConfDir}";
+
+        unitConfig.ConditionPathExists = [ "/proc/acpi" ];
       };
 
   };