flake: don't use self in let block

The upcoming change factors out a lot of code to make it reusable for a
"legacy" Hydra project that we use to build a matrix of (Lix x nixpkgs)
to have a warm cache for everyone.

The Hydra project will not use flakes to have some inputs overridable,
so the change is needed. In non-flake mode we don't have a notion of
"self", so rename it to `lixSrc` such that the next commit is purely
about moving stuff without changing any aspect.

Current stock Hydra doesn't expose `lastModified` though, so right now
that is a cache-miss. There's a pending Hydra PR fixing that[1].

[1] https://git.lix.systems/lix-project/hydra/pulls/82

Change-Id: I59d70b8b874f576c782217e7bca099eda4c7add9

flake.nix

@@ -42,6 +42,7 @@
 
     let
       inherit (nixpkgs) lib;
+      lixSrc = self;
 
       # This notice gets echoed as a dev shell hook, and can be turned off with
       # `touch .nocontribmsg`
@@ -87,8 +88,8 @@
           ""
         else
           "pre${
-            builtins.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")
-          }-dev_${self.shortRev or "dirty"}";
+            builtins.substring 0 8 (lixSrc.lastModifiedDate or lixSrc.lastModified or "19700101")
+          }-dev_${lixSrc.shortRev or "dirty"}";
 
       linux32BitSystems = [ "i686-linux" ];
       linux64BitSystems = [