Re-add flake.lock workflow from #130, with only manual dispatch

flake.lock needs to be updated for Go 1.19!

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

@@ -0,0 +1,32 @@
+name: "Update flake.lock"
+on:
+  workflow_dispatch:
+
+jobs:
+  update-lock:
+    name: lock
+    runs-on: ubuntu-latest
+    steps:
+
+      - name: Checkout repo
+        uses: actions/checkout@v2
+
+      - name: Install nix
+        uses: cachix/install-nix-action@v17
+        with:
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+          nix_path: nixpkgs=channel:nixos-unstable
+        env:
+          NIX_CONFIG: access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+
+      - name: Update flake deps
+        run: nix flake update
+        env:
+          NIX_CONFIG: access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+
+      - name: Commit
+        uses: stefanzweifel/git-auto-commit-action@v4
+        with:
+          commit_message: "chore: update flake.lock"
+          branch: master