Add CI job to verify that the action works

.envrc

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+use flake

.github/workflows/ci.yaml

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+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  run-x86_64-linux:
+    name: Run x86_64 Linux
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v3
+      - name: Check flake.lock
+        uses: ./
+
+  run-x86_64-darwin:
+    name: Run x86_64 Darwin
+    runs-on: macos-12
+    steps:
+      - uses: actions/checkout@v3
+      - name: Check flake.lock
+        uses: ./

flake.lock

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+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1685836261,
+        "narHash": "sha256-rpxEPGeW4JZJcH58SQApJUtJ7w78VPtkF6Cut/Pq6Kg=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "dd4982554e18b936790da07c4ea2db7c7600f283",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

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+# This flake.nix is only here to provide a flake.lock to check in CI
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs, ... }:
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forAllSystems = f: nixpkgs.lib.genAttrs supportedSystems (system: f rec {
+        pkgs = import nixpkgs { inherit system; };
+      });
+    in
+    {
+      devShells = forAllSystems ({ pkgs }: {
+        default =
+          pkgs.mkShell {
+            packages = (with pkgs; [
+              nixpkgs-fmt
+            ]);
+          };
+      });
+    };
+}