Switch CI to Nix flakes

We also move tests to Flakes.

This would allow users to submit PRs with a fork of nixpkgs when they
want to test nixpkgs PRs against SNM.

flake.nix

@@ -4,15 +4,55 @@
   inputs = {
     utils.url = "github:numtide/flake-utils";
     nixpkgs.url = "flake:nixpkgs/nixos-unstable";
+    nixpkgs-21_05.url = "flake:nixpkgs/nixos-21.05";
+    blobs = {
+      url = "gitlab:simple-nixos-mailserver/blobs";
+      flake = false;
+    };
   };
 
-  outputs = { self, utils, nixpkgs }: {
+  outputs = { self, utils, blobs, nixpkgs, nixpkgs-21_05 }: let
+    system = "x86_64-linux";
+    pkgs = nixpkgs.legacyPackages.${system};
+    # We want to test nixos-mailserver on several nixos releases
+    releases = [
+      {
+        name = "unstable";
+        pkgs = nixpkgs.legacyPackages.${system};
+      }
+      {
+        name = "21_05";
+        pkgs = nixpkgs-21_05.legacyPackages.${system};
+      }
+    ];
+    testNames = [
+      "internal"
+      "external"
+      "clamav"
+      "multiple"
+    ];
+    genTest = testName: release: {
+      "name"= "${testName}-${release.name}";
+      "value"= import (./tests/. + "/${testName}.nix") {
+        pkgs = release.pkgs;
+        inherit blobs;
+      };
+    };
+    # Generate an attribute set such as
+    # {
+    #   external-unstable = <derivation>;
+    #   external-21_05 = <derivation>;
+    #   ...
+    # }
+    allTests = pkgs.lib.listToAttrs (
+      pkgs.lib.flatten (map (t: map (r: genTest t r) releases) testNames));
+
+  in {
     nixosModules.mailserver = import ./.;
     nixosModule = self.nixosModules.mailserver;
-  } // utils.lib.eachDefaultSystem (system: let
-    pkgs = nixpkgs.legacyPackages.${system};
-  in {
-    devShell = pkgs.mkShell {
+    hydraJobs.${system} = allTests;
+    checks.${system} = allTests;
+    devShell.${system} = pkgs.mkShell {
       buildInputs = with pkgs; [
         (python3.withPackages (p: with p; [
           sphinx
@@ -22,5 +62,5 @@
         clamav
       ];
     };
-  });
+  };
 }