Add task to update pixi weekly

.github/workflows/ci.yml

@@ -1,5 +1,23 @@
 name: MODFLOW 6 continuous integration
 on:
+  push:
+    branches: [master, develop, update/pixi-lock]
+    tags: ["*"]
+    paths-ignore:
+      - '**.md'
+      - '**.pdf'
+      - '**.tex'
+      - '**.jpg'
+      - '**.jpeg'
+      - '**.png'
+      - '**.bbl'
+      - '**.bib'
+      - 'doc/**.dat'
+      - 'doc/**.ipynb'
+      - 'doc/**.py'
+      - 'doc/**.sh'
+      - 'doc/**.xlsx'
+      - '.hpc/**'
   pull_request:
     branches:
       - master

.github/workflows/pixi_auto_update.yml

@@ -0,0 +1,32 @@
+name: Pixi auto update
+
+on:
+  schedule:
+  # At 03:00 on day 3 of the month
+    - cron: "0 3 3 * *"
+  # on demand
+  workflow_dispatch:
+
+jobs:
+  auto-update:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ssh-key: ${{ secrets.SSH_PRIVATE_KEY }}
+      - uses: prefix-dev/setup-pixi@v0.5.1
+        with:
+          pixi-version: "latest"
+          cache: false
+      - name: Update pixi lock file
+        run: |
+          rm pixi.lock
+          pixi install
+      - uses: peter-evans/create-pull-request@v6
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          branch: update/pixi-lock
+          title: Update pixi lock file
+          commit-message: "Update `pixi.lock`"
+          body: Update pixi dependencies to the latest version.
+          author: "GitHub <noreply@github.com>"