unifi, nixos/unifi: support LTS (5.6.36) and release (5.7.20)

Ubiquiti has both a LTS and current version of their Unifi controller software.

The latter adds new features, but may drop support for some devices.

This adds the capability to use either for the unifi module but defaults
to the LTS version, which was the previous behavior.

pkgs/servers/unifi/default.nix

@@ -2,21 +2,37 @@
 , dpkg
 , fetchurl
 , unzip
+, useLTS ? false
 }:
 
-stdenv.mkDerivation rec {
-  name = "unifi-controller-${version}";
-  version = "5.6.36";
 
-  src = fetchurl {
+let
+  versions = {
+    stable = {
+      version = "5.7.20";
+      sha256 = "1ylj4i5mcv6z9n32275ccdf1rqk74zilqsih3r6xzhm30pxrd8dd";
+    };
+    lts = {
+      version = "5.6.36";
+      sha256 = "075q7vm56fdsjwh72y2cb1pirl2pxdkvqnhvd3bf1c2n64mvp6bi";
+    };
+  };
+  selectedVersion =
+    let attr = if useLTS then "lts" else "stable";
+    in versions."${attr}";
+in
+
+stdenv.mkDerivation {
+  name = "unifi-controller-${selectedVersion.version}";
+  src = with selectedVersion; fetchurl {
     url = "https://dl.ubnt.com/unifi/${version}/unifi_sysvinit_all.deb";
-    sha256 = "075q7vm56fdsjwh72y2cb1pirl2pxdkvqnhvd3bf1c2n64mvp6bi";
+    inherit sha256;
   };
 
   buildInputs = [ dpkg ];
 
   unpackPhase = ''
-    dpkg-deb -x ${src} ./
+    dpkg-deb -x $src ./
   '';
 
   doConfigure = false;