feat(shell): Add an action to cache it

.forgejo/workflows/eval-shell.yaml

@@ -0,0 +1,19 @@
+jobs:
+  build-shell:
+    runs-on: nix
+    steps:
+    - uses: actions/checkout@v3
+    - env:
+        STORE_ENDPOINT: https://tvix-store.dgnum.eu/infra-signing/
+        STORE_PASSWORD: ${{ secrets.STORE_PASSWORD }}
+        STORE_USER: admin
+      name: Build and cache shell
+      run: nix-shell -A eval-shell --run "nix-cache -A devShell"
+name: Build the shell
+on:
+  pull_request:
+    branches:
+    - main
+  push:
+    branches:
+    - main