Add a nix/build action

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

.forgejo/workflows/nix-build.yaml

@@ -0,0 +1,36 @@
+---
+name: test nix/build
+
+on:
+  push:
+
+jobs:
+  nix/build:
+    runs-on: nixos-flakes
+    steps:
+      - name: checkout
+        uses: actions/checkout@v3
+
+      - name: build the local test package
+        id: build-local
+        uses: ./build
+        with:
+          flake: ./build/test
+          package: "hello"
+          logs: true
+
+      - name: print the output path
+        run: |
+          echo ${{ steps.build-local.outputs.output-path }}
+
+      - name: build a package from nixpkgs
+        id: build-nixpkgs
+        uses: ./build
+        with:
+          flake: nixpkgs
+          package: "hello"
+          logs: true
+
+      - name: print the output path
+        run: |
+          echo ${{ steps.build-nixpkgs.outputs.output-path }}