Expose option to configure branch for PR (#36)

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/83658b28fe638a170a19b8933aa008b30640fbd1' (2022-05-26)
  → 'github:nixos/nixpkgs/e0169d7a9d324afebf5679551407756c77af8930' (2022-06-08)

README.md

@@ -94,6 +94,36 @@ jobs:
         run: echo Pull request number is ${{ steps.update.outputs.pull-request-number }}.
 ```
 
+## Example that doesn't run on PRs
+
+If you were to run this action as a part of your CI workflow, you may want to prevent it from running against Pull Requests.
+
+```yaml
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  pull_request: # triggers on every Pull Request
+  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
+        if: ${{ github.event_name != 'pull_request' }}
+        uses: DeterminateSystems/update-flake-lock@vX
+        with:
+          inputs: input1 input2 input3
+```
+
 ## Running GitHub Actions CI
 
 GitHub Actions will not run workflows when a branch is pushed by or a PR is opened by a GitHub Action. There are two ways to have GitHub Actions CI run on a PR submitted by this action.

action.yml

@@ -13,6 +13,10 @@ inputs:
     description: 'The message provided with the commit'
     required: false
     default: "flake.lock: Update"
+  branch:
+    description: 'The branch of the PR to be created'
+    required: false
+    default: "update_flake_lock_action"
   pr-title:
     description: 'The title of the PR to be created'
     required: false
@@ -49,7 +53,7 @@ runs:
       id: create-pr
       uses: peter-evans/create-pull-request@v3
       with:
-        branch: update_flake_lock_action
+        branch: ${{ inputs.branch }}
         delete-branch: true
         title: ${{ inputs.pr-title }}
         token: ${{ inputs.token }}

flake.lock

@@ -2,16 +2,16 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1638263381,
-        "narHash": "sha256-1rZDxTw74ETuJEjwPfpMgY0sfx8Cv1tRNt3gibol574=",
+        "lastModified": 1654682581,
+        "narHash": "sha256-Jb1PQCwKgwdNAp907eR5zPzuxV+kRroA3UIxUxCMJ9s=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "7b031d0d99e8cdaf0b70457c0cb33f16c0c958bb",
+        "rev": "e0169d7a9d324afebf5679551407756c77af8930",
         "type": "github"
       },
       "original": {
         "owner": "nixos",
-        "ref": "nixos-unstable-small",
+        "ref": "nixos-unstable",
         "repo": "nixpkgs",
         "type": "github"
       }