actions: Add basic github actions

.envrc

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+use nix

.github/workflows/codestyle.yml

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+---
+name: Codestyle checks
+
+'on':
+  pull_request:
+    branches:
+      - main
+
+  push:
+    branches:
+      - main
+      - nodejs-frontend  # TODO: Remove me when web-frontend is merged
+
+jobs:
+  yamllint:
+    name: Run yamllint on github actions files
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/[email protected]
+
+      - uses: cachix/install-nix-action@v13
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+      - name: Prefetch shell.nix
+        run: 'nix-shell --run true'
+
+      - run: nix-shell --run "make yamllint"

Makefile

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+yamllint:
+	yamllint -s -f github .github/workflows/*

shell.nix

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+{ pkgs ? (import <nixpkgs> {}), ... }:
+
+pkgs.mkShell {
+  buildInputs = [
+    # Install GNU Make for shorthands
+    pkgs.gnumake
+
+    # Install yaml lint
+    pkgs.yamllint
+  ];
+}