Add a nix/build action

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

build/action.yml

@@ -0,0 +1,30 @@
+---
+name: nix-build
+author: Gergely Nagy
+description: "Build packages with Nix"
+inputs:
+  flake:
+    description: "The flake to build packages from"
+    default: "."
+    required: true
+  package:
+    description: "The package to build from a flake"
+    required: false
+  logs:
+    description: "Enable showing the full build logs"
+    default: false
+    required: false
+outputs:
+  output-path:
+    description: "The resulting output paths"
+    value: "${{ steps.nix-build.outputs.output-path }}"
+runs:
+  using: "composite"
+  steps:
+    - id: nix-build
+      shell: bash
+      run: $GITHUB_ACTION_PATH/bin/nix-build
+      env:
+        INPUT_FLAKE: ${{ inputs.flake }}
+        INPUT_PACKAGE: ${{ inputs.package }}
+        INPUT_LOGS: ${{ inputs.logs }}