nixos/networkd: add ipv4-fallback and fallback as valid options t…

…o `LinkLocalAddressing`

Both options were introduced in systemd v243[1]. Those options can be
used to ensure that LinkLocalAddressing is only configured for a given
interface if DHCPv4 fails. To quote `systemd.network(5)`:

```
If "fallback" or "ipv4-fallback" is specified, then an IPv4
link-local address is configured only when DHCPv4 fails. If "fallback", an IPv6 link-local
address is always configured, and if "ipv4-fallback", the address is not configured. Note
that, the fallback mechanism works only when DHCPv4 client is enabled, that is, it requires
"DHCP=yes" or "DHCP=ipv4".
```

[1] systemd/systemd@8bc17bb

nixos/modules/system/boot/networkd.nix

@@ -187,7 +187,7 @@ let
     # Note: For DHCP the values both, none, v4, v6 are deprecated
     (assertValueOneOf "DHCP" ["yes" "no" "ipv4" "ipv6" "both" "none" "v4" "v6"])
     (assertValueOneOf "DHCPServer" boolValues)
-    (assertValueOneOf "LinkLocalAddressing" ["yes" "no" "ipv4" "ipv6"])
+    (assertValueOneOf "LinkLocalAddressing" ["yes" "no" "ipv4" "ipv6" "ipv4-fallback" "fallback"])
     (assertValueOneOf "IPv4LLRoute" boolValues)
     (assertValueOneOf "LLMNR" ["yes" "resolve" "no"])
     (assertValueOneOf "MulticastDNS" ["yes" "resolve" "no"])