Pin haskell.nix

When it changes we often have to rebuild ghc ... :(

nixos/modules/leasereport.nix

 { callPackage }:
 let
   leasereport = import ./leasereport-repo.nix;
+
+  # Pin a particular version of haskell.nix.  The particular version isn't
+  # special.  It's just recent at the time this expression was written and it
+  # is known to work with LeaseReport.  It could be bumped if necessary but
+  # this would probably only happen as a result of bumping the resolver in
+  # stack.yaml.
+  haskellNixSrc ? builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/31fd01e14729e686de0a915b9bc4ff70397a84c7.tar.gz
 in
-  (callPackage "${leasereport}/nix" { }).LeaseReport.components.exes.LeaseReport
+  (callPackage "${leasereport}/nix" { inherit haskellNixSrc; }).LeaseReport.components.exes.LeaseReport