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.lock

@@ -1,5 +1,21 @@
 {
   "nodes": {
+    "blobs": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1604995301,
+        "narHash": "sha256-wcLzgLec6SGJA8fx1OEN1yV/Py5b+U5iyYpksUY/yLw=",
+        "owner": "simple-nixos-mailserver",
+        "repo": "blobs",
+        "rev": "2cccdf1ca48316f2cfd1c9a0017e8de5a7156265",
+        "type": "gitlab"
+      },
+      "original": {
+        "owner": "simple-nixos-mailserver",
+        "repo": "blobs",
+        "type": "gitlab"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1607522989,
@@ -15,9 +31,26 @@
         "type": "indirect"
       }
     },
+    "nixpkgs-21_05": {
+      "locked": {
+        "lastModified": 1625692408,
+        "narHash": "sha256-e9L3TLLDVIJpMnHtiNHJE62oOh6emRtSZ244bgYJUZs=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c06613c25df3fe1dd26243847a3c105cf6770627",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-21.05",
+        "type": "indirect"
+      }
+    },
     "root": {
       "inputs": {
+        "blobs": "blobs",
         "nixpkgs": "nixpkgs",
+        "nixpkgs-21_05": "nixpkgs-21_05",
         "utils": "utils"
       }
     },