Automate cargo update without dependabot (rust-lang#2942)

Automatically create pull requests from the result of running `cargo
update` every Monday morning. This should avoid the need for manual PRs
to update Cargo.lock, which seemingly dependabot wouldn't take care of.
We now only use dependabot to update github actions.

This revives what I had initially proposed in rust-lang#2895 in light of rust-lang#2940.

.github/dependabot.yml

@@ -9,13 +9,3 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
-
-  - package-ecosystem: "cargo"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    groups:
-      cargo:
-        update-types:
-          - "minor"
-          - "patch"

.github/workflows/cargo-update.yml

@@ -0,0 +1,47 @@
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+
+name: Attempt cargo update
+
+on:
+  schedule:
+    - cron: "30 3 * * Mon" # Run this every Monday at 03:30 UTC
+  workflow_dispatch:     # Allow manual dispatching for a custom branch / tag.
+
+permissions:
+  checks: write
+  contents: write
+  pull-requests: write
+
+jobs:
+  create-cargo-update-pr:
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout Kani
+        uses: actions/checkout@v3
+
+      - name: Setup Kani Dependencies
+        uses: ./.github/actions/setup
+        with:
+          os: ubuntu-22.04
+
+      - name: Run cargo update
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          today=$(date +%Y-%m-%d)
+          echo "today=$today" >> $GITHUB_ENV
+          if ! git ls-remote --exit-code origin cargo-update-$today ; then
+            cargo update
+            cargo build-dev
+            git diff
+          fi
+      - name: Create Pull Request
+        uses: peter-evans/create-pull-request@v5
+        with:
+          commit-message: Upgrade cargo dependencies to ${{ env.today }}
+          branch: cargo-update-${{ env.today }}
+          delete-branch: true
+          title: 'Automatic cargo update to ${{ env.today }}'
+          body: >
+            Dependency upgrade resulting from `cargo update`.