Use the nix command (and flakes) in the CI

Apart from a slight simplification and a bit of dogfooding, this also
make the cache behavior more predictable.
For example `nix build .` and `nix build nix/$(git rev-parse HEAD)` will
yield the exact same path, while their “intuitive” non-flake equivalents
(`nix-build` and
`nix-build https://github.com/nixos/nix/archives/$(git rev-parse HEAD).tar.gz`)
don’t.

This was a pain for example in #5059

Also, the `bar-with-logs` log format is imho nicer (even in an
non-interactive context) because prefixing each log line with the name
of the derivation that produced it makes it much easier to follow what’s
going on.

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
  name: '${{ env.CACHIX_NAME }}'
  signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
  authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
- - run: nix-build -A checks.$(nix-instantiate --eval -E '(builtins.currentSystem)')
+ - run: nix --experimental-features 'nix-command flakes' flake check -L
 
  check_cachix:
  name: Cachix secret present for installer tests
@@ -95,7 +95,7 @@ jobs:
  name: '${{ env.CACHIX_NAME }}'
  signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
  authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
- - run: nix-build -A checks.$(nix-instantiate --eval -E 'builtins.currentSystem' --json).dockerImage
+ - run: nix --experimental-features 'nix-command flakes' build .#dockerImage -L
  - run: docker load -i ./result/image.tar.gz
  - run: docker tag nix:$NIX_VERSION nixos/nix:$NIX_VERSION
  - run: docker tag nix:$NIX_VERSION nixos/nix:master

flake.nix

@@ -447,19 +447,7 @@
  installerScriptForGHA = installScriptFor [ "x86_64-linux" "x86_64-darwin" "armv6l-linux" "armv7l-linux"];
 
  # docker image with Nix inside
- dockerImage = nixpkgs.lib.genAttrs linux64BitSystems (system:
- let
- pkgs = nixpkgsFor.${system};
- image = import ./docker.nix { inherit pkgs; tag = version; };
- in pkgs.runCommand "docker-image-tarball-${version}"
- { meta.description = "Docker image with Nix for ${system}";
- }
- ''
- mkdir -p $out/nix-support
- image=$out/image.tar.gz
- ln -s ${image} $image
- echo "file binary-dist $image" >> $out/nix-support/hydra-build-products
- '');
+ dockerImage = nixpkgs.lib.genAttrs linux64BitSystems (system: self.packages.${system}.dockerImage);
 
  # Line coverage analysis.
  coverage =
@@ -606,6 +594,20 @@
 
  hardeningDisable = [ "pie" ];
  };
+ dockerImage =
+ let
+ pkgs = nixpkgsFor.${system};
+ image = import ./docker.nix { inherit pkgs; tag = version; };
+ in
+ pkgs.runCommand
+ "docker-image-tarball-${version}"
+ { meta.description = "Docker image with Nix for ${system}"; }
+ ''
+ mkdir -p $out/nix-support
+ image=$out/image.tar.gz
+ ln -s ${image} $image
+ echo "file binary-dist $image" >> $out/nix-support/hydra-build-products
+ '';
  } // builtins.listToAttrs (map (crossSystem: {
  name = "nix-${crossSystem}";
  value = let