refactor(ci): auto-discover nixosConfigurations from systems directory

Replace the hardcoded list of systems in flake.nix with automatic
discovery of host directories under ./systems, and generate the CI
build matrix dynamically by evaluating nixosConfigurations rather
than maintaining a duplicate list in the workflow.

.github/workflows/build_systems.yml

@@ -8,16 +8,28 @@ on:
     - cron: "0 22 * * *"
 
 jobs:
+  matrix:
+    name: generate-matrix
+    runs-on: self-hosted
+    outputs:
+      systems: ${{ steps.systems.outputs.systems }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Collect nixosConfigurations
+        id: systems
+        run: |
+          systems="$(nix eval --accept-flake-config --json \
+            '.#nixosConfigurations' \
+            --apply 'builtins.attrNames')"
+          echo "systems=$systems" >> "$GITHUB_OUTPUT"
+
   build:
     name: build-${{ matrix.system }}
+    needs: matrix
     runs-on: self-hosted
     strategy:
       matrix:
-        system:
-          - "bob"
-          - "brain"
-          - "jeeves"
-          - "rhapsody-in-green"
+        system: ${{ fromJSON(needs.matrix.outputs.systems) }}
     continue-on-error: true
     steps:
       - uses: actions/checkout@v4

flake.nix

@@ -95,37 +95,18 @@
         }
       );
 
-      nixosConfigurations = {
-        bob = lib.nixosSystem {
-          modules = [
-            ./systems/bob
-          ];
-          specialArgs = { inherit inputs outputs; };
-        };
-        brain = lib.nixosSystem {
-          modules = [
-            ./systems/brain
-          ];
-          specialArgs = { inherit inputs outputs; };
-        };
-        jeeves = lib.nixosSystem {
-          modules = [
-            ./systems/jeeves
-          ];
-          specialArgs = { inherit inputs outputs; };
-        };
-        rhapsody-in-green = lib.nixosSystem {
-          modules = [
-            ./systems/rhapsody-in-green
-          ];
-          specialArgs = { inherit inputs outputs; };
-        };
-        iso = lib.nixosSystem {
-          modules = [
-            ./systems/iso
-          ];
-          specialArgs = { inherit inputs outputs; };
-        };
-      };
+      nixosConfigurations =
+        let
+          hosts = builtins.attrNames (
+            lib.filterAttrs (_: type: type == "directory") (builtins.readDir ./systems)
+          );
+          mkHost =
+            name:
+            lib.nixosSystem {
+              modules = [ ./systems/${name} ];
+              specialArgs = { inherit inputs outputs; };
+            };
+        in
+        lib.genAttrs hosts mkHost;
     };
 }