Rework the CI

This reworks the CI to give more control to GitHub Actions.

- The flake exposes the list of smoke-test derivations that GHA load and
  execute.
- The repositories for base (available only for PRs) and head are built
  concurrently
- Built repositories are cached by their commit hash, we should not need
  to build them more than once. We can always delete the specific cache
  in case of issues.

Minor changes:
- haskellPackages -> chapPackages
- Consistent camelCase
- chapPackagesLatestVersion maps to a single version rather than a list
  with a single element

Bump flake

.github/actions/build-repo/action.yml

@@ -0,0 +1,33 @@
+name: Build repository
+inputs:
+  ref:
+    required: true
+runs:
+  using: composite
+  steps:
+    - name: Checkout ${{ inputs.ref }}
+      uses: actions/checkout@v4
+      with:
+        ref: ${{ inputs.ref }}
+
+    - name: Unpack keys
+      env:
+        KEYS: ${{ secrets.KEYS }}
+      run: |
+        if [[ -z "$KEYS" ]]; then
+          echo "Could not access repository secret keys (PR is coming from a fork?)"
+          echo "Generating fresh keys for this run"
+          nix develop -c foliage create-keys
+        else
+          mkdir _keys
+          echo "$KEYS" | base64 -d | tar xvz -C _keys
+        fi
+
+    - name: Build repository
+      run: nix develop --command foliage build -j 0  --write-metadata
+
+    - name: Copy static web assets
+      run: |
+        cp static/index.html _repo
+        cp README.md _repo
+