nixos: support arbitrary child binaries

Allow child binaries to be ERIS URNs or otherwise arbitrary strings.

nixos-modules/lib/children.nix

@@ -16,9 +16,7 @@
                 description =
                   "Program binary for this child. Must be an ERIS URN.";
                 default = null;
-                type = types.nullOr (types.str // {
-                  check = lib.strings.hasPrefix "urn:erisx2:";
-                });
+                type = types.nullOr types.str;
                 example =
                   "urn:erisx2:AEAU4KT7AGJLA5BHPWFZ7HX2OVVNVFGDM2SIS726OPZBGXDED64QIDPHN2M5P5HIMOG3YDSWBGDPNUMZKCG4CRVU4DI5BOS5IJRFCSLQQY";
               };