Update flake inputs (and create PRs) weekly

This new github actions workflow will update (once a week) flake inputs
and create a PR.

It will update all inputs, except `nixpkgs-wasm` since that input needs
to be fixed (at least for now).

.github/workflows/update-flake-lock.yml

@@ -0,0 +1,21 @@
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '0 0 * * 0' # runs weekly on Sunday at 00:00
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Install Nix
+        uses: cachix/install-nix-action@v16
+        with:
+          extra_nix_config: |
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@v3
+        with:
+          inputs: nixpkgs nixpkgs-mozilla import-cargo