flake-update: init action

LICENSE

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+MIT License
+
+Copyright (c) 2021 Determinate Systems, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# update-flake-lock
+
+This is a GitHub Action that will update your flake.lock file whenever it is run.
+
+> NOTE: We hardcode the `install_url` to a relatively recent `nixUnstable` (`nix-2.5pre20211015_130284b` currently). If you need a newer version that includes a new feature or important bug fix, feel free to file an issue or send a PR bumping the `install_url` inside the [`action.yml`](action.yml)!
+
+## Example
+
+An example GitHub Action workflow using this action would look like the following:
+
+```yaml
+name: update-flake-lock
+on:
+  workflow_dispatch:
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@main
+```
+
+To have this workflow run on a recurring basis, see the [GitHub Actions documentation on the `schedule` key](https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#schedule).

action.yml

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+name: 'Update flake.lock'
+description: 'Update your flake.lock and send a PR'
+runs:
+  using: "composite"
+  steps:
+    - uses: cachix/install-nix-action@v14
+      with:
+        install_url: https://github.com/numtide/nix-unstable-installer/releases/download/nix-2.5pre20211015_130284b/install
+        extra_nix_config: |
+          experimental-features = nix-command flakes
+    - run: nix flake lock --commit-lock-file
+      shell: bash
+      env:
+        GIT_AUTHOR_NAME: github-actions[bot]
+        GIT_AUTHOR_EMAIL: <github-actions[bot]@users.noreply.github.com>
+        GIT_COMMITTER_NAME: github-actions[bot]
+        GIT_COMMITTER_EMAIL: <github-actions[bot]@users.noreply.github.com>
+    - name: Create PR
+      uses: peter-evans/create-pull-request@v3
+      with:
+        branch: update_flake_lock_action
+        delete-branch: true
+        title: "flake.lock: Update"
+        body: Automated changes by the [update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock) GitHub Action.