Convert to Nix flakes (#20)

https://nixos.wiki/wiki/Flakes

"Flakes" don't assume impure global nixpkgs, rather refer to them
directly via the combination of `inputs` in `flake.nix` and the
`flake.lock` file.

- `default.nix` renamed to `build.nix` with minimal changes
- the import of `tt-rss.nix` was moved into the flake 

github action changes:
- bump to v24
- no need for nix_path, since the flake refers to the exact `nixpkgs`
- added `flake check`
- flakes are automatically supported by the action, so no changes were
required

.github/workflows/ci.yml

@@ -17,14 +17,12 @@ jobs:
   ci:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
-    - uses: cachix/install-nix-action@v22
-      with:
-        nix_path: nixpkgs=channel:nixos-23.05
+    - uses: cachix/install-nix-action@v24
 
-    # build without systemd support for now, since the closure becomes larger
-    - run: nix-build --arg withSystemd false
+    - run: nix build --no-update-lock-file
+    - run: nix flake check --no-update-lock-file
 
     - name: Prepare artifacts
       run: |

default.nix → build.nix

@@ -1,9 +1,7 @@
-{ pkgs ? import <nixpkgs> {}, withSystemd ? true }:
+{ pkgs, ttRss, withSystemd ? true }:
 
 let
 
-  ttRss = (import ./tt-rss.nix { inherit pkgs; });
-
   php = (pkgs.php.override {
     embedSupport = true;
     cliSupport = true;

flake.nix

@@ -0,0 +1,13 @@
+{
+  description = ''Portable "Tiny Tiny Rss" service run by uwsgi-php and built with Nix'';
+
+  inputs.nixpkgs.url = github:NixOS/nixpkgs/nixos-23.11;
+
+  outputs = { self, nixpkgs, ... }:
+    let
+      pkgs = import nixpkgs { system = "x86_64-linux"; };
+      ttRss = (import ./tt-rss.nix { inherit pkgs; });
+    in {
+      packages.x86_64-linux.default = (import ./build.nix { inherit pkgs ttRss; withSystemd = false; });
+    };
+}