add github action

.github/workflows/update.yml

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+name: Update
+run-name: ${{github.actor}} is updating capkgs
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+  pull_request:
+permissions:
+  contents: read
+  pull-requests: read
+concurrency:
+  group: '${{ github.workflow }} @ ${{ github.event.pull_request.head.label || github.head_ref || github.ref }}'
+  cancel-in-progress: true
+jobs:
+  update:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: cachix/install-nix-action@v23
+        with:
+          extra_nix_config: |
+            trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+            substituters = https://cache.iog.io https://cache.nixos.org/
+      - run: nix develop --ignore-environment --command just packages
+      - run: nix develop --ignore-environment --command just check

.gitignore

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+/evals
+/closures
+/builds