github: remove workflows

Since Hercules CI is in use now, GitHub Actions are not necessarily required

.github/workflows/build.yml

@@ -1,12 +0,0 @@
-name: build
-on: [pull_request, push]
-jobs:
-  tests:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v19
-      with:
-        github_access_token: ${{ secrets.GITHUB_TOKEN }}
-    - run: nix build
-    - run: nix flake check