nix: Build on GHC 8.8 as well

We are using a different nixpkgs only because that's what default.nix
has been using.

.github/workflows/ci.yaml

@@ -13,3 +13,5 @@ jobs:
           experimental-features = nix-command flakes
     - name: Build and test 🔧
       run: nix build -j 4 -L
+    - name: Build and test (GHC 8.8) 🔧
+      run: nix build -j 4 -L .#jrec-ghc88

flake.lock

@@ -34,6 +34,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-ghc88": {
+      "locked": {
+        "lastModified": 1596466290,
+        "narHash": "sha256-V1vrlYfVEdW4iauUc58LCtorSxLLF+X/PhjD5+dQjeQ=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "76f2e271a2ef9de3734dcc9d366a42d6bfb18e82",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "76f2e271a2ef",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs-lib": {
       "locked": {
         "dir": "lib",
@@ -55,7 +71,8 @@
     "root": {
       "inputs": {
         "flake-parts": "flake-parts",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "nixpkgs-ghc88": "nixpkgs-ghc88"
       }
     }
   },

flake.nix

@@ -1,6 +1,7 @@
 {
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    nixpkgs-ghc88.url = "github:nixos/nixpkgs/76f2e271a2ef";
     flake-parts.url = "github:hercules-ci/flake-parts";
   };
   outputs = inputs@{ nixpkgs, flake-parts, ... }:
@@ -11,17 +12,22 @@
           overlay = self: super: {
             jrec = self.callCabal2nix "jrec" ./. { };
           };
-          haskellPackages' = pkgs.haskellPackages.extend overlay;
+          ghcVersions = {
+            ghc88 = inputs.nixpkgs-ghc88.legacyPackages.${system}.haskellPackages.extend overlay;
+            ghc92 = pkgs.haskellPackages.extend overlay;
+          };
         in
         {
-          packages.default = haskellPackages'.jrec;
-          devShells.default = haskellPackages'.shellFor {
+          packages.default = ghcVersions.ghc92.jrec;
+          devShells.default = ghcVersions.ghc92.shellFor {
             packages = p: [ p.jrec ];
-            buildInputs = with haskellPackages'; [
+            buildInputs = with ghcVersions.ghc92; [
               cabal-install
               haskell-language-server
             ];
           };
+          # Expose jrec built with GHC 8.8 so it can be tested in CI.
+          packages.jrec-ghc88 = ghcVersions.ghc88.jrec;
         };
     };
 }