detect nix flakes config

[blaggacao]

• feat: read out flake config if a flake
• feat: make a flakes variant
• docs: document the new flakes variant

[rickynils]

Zooming the use case still one step out, I have a good idea for opt-in:

uses: nixbuild/nix-quick-install-action@v22-flake

That's fully specified intent at the lowest verbosity level and would include this PR's capabilities.

I'm not sold on the idea of a different tag for this. It is bad for discoverability, and makes maintenance a bit worse (we need to create the extra tags, although I guess it can be automated).

But we could have a setting that, when turned on, would both add the necessary stuff ('nix-commands flakes') and do the config evaluation. Essentially what you have now, except checking an input instead of the ref.

[blaggacao]

Enable the experimental features by default (if your Nix version is new enough). Enable config evaluation by default, but add an option for opting-out (for the really time-concerned people, or as a quick fix if it turns out the config evaluation messes up some specific use case).

Barring some more testing, this is now prototypically implemented.

Can you advise on writing/ameding test cases?

[rickynils]

@blaggacao Maybe something like this can be used for testing it:

diff --git a/.github/workflows/cicd.yml b/.github/workflows/cicd.yml
index 145a654..05e977d 100644
--- a/.github/workflows/cicd.yml
+++ b/.github/workflows/cicd.yml
@@ -251,6 +251,22 @@ jobs:
           stat ~/.cache/nix/test-path
           nix-store --verify-path ~/.cache/nix/test-path
 
+  test-nix-config-eval:
+    needs: build
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/download-artifact@v3
+        id: nix-archives
+        with:
+          name: nix-archives-${{ runner.os }}
+      - uses: ./
+        with:
+          nix_archives_url: file://${{steps.nix-archives.outputs.download-path}}
+      - name: Verify Nix config
+        run: |
+          nix show-config | grep -q 'stalled-download-timeout = 333'
+
   release:
     runs-on: ubuntu-latest
     needs:
diff --git a/flake.nix b/flake.nix
index bc794ac..a29d581 100644
--- a/flake.nix
+++ b/flake.nix
@@ -15,6 +15,12 @@
     nixpkgs-nix-2_0_4.flake = false;
   };
 
+  nixConfig = {
+    # We set some dummy Nix config here so we can use it for verification in our
+    # CI test
+    stalled-download-timeout = 333; # default 300
+  }
+
   outputs = {
     self,
     flake-utils,