STR-676: sets a CI autoupdate cron job

.github/workflows/cron-nightly-rust.yml

@@ -0,0 +1,57 @@
+name: Update nightly Rust
+on:
+  schedule:
+    - cron: "29 17 1 * *" # At 17:29 on day-of-month 1.
+  workflow_dispatch: # allows manual triggering
+env:
+  CARGO_TERM_COLOR: always
+jobs:
+  format:
+    name: Update nightly Rustc
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - uses: dtolnay/rust-toolchain@nightly
+      - name: Update rust-toolchain.toml to use latest nightly
+        run: |
+          set -xe
+
+          # Get the latest nightly date.
+          # If the latest nightly date is not the same as the current nightly date,
+          # rustup will default to the latest nightly up to TODAY.
+          #
+          # Example: If the latest nightly is 2023-07-01, and the TODAY is 2023-07-02,
+          # then rustup will default to nightly-2023-07-01.
+          TODAY="$(date +%Y-%m-%d)"
+
+          # Update the nightly version in the rust-toolchain.toml file.
+          echo "Updating rust-toolchain.toml to use nightly-$TODAY"
+          sed -i "s/^channel = \"nightly-.*\"/channel = \"nightly-$TODAY\"/" rust-toolchain.toml
+
+          # Update the nightly date in the environment.
+          echo "nightly_date=${TODAY}" >> "$GITHUB_ENV"
+
+          # Maybe there is no new nightly.
+          # In this case don't make an empty PR.
+          if ! git diff --exit-code > /dev/null; then
+              echo "Updated nightly. Opening PR."
+              echo "changes_made=true" >> "$GITHUB_ENV"
+          else
+              echo "Attempted to update nightly but the latest-nightly date did not change. Not opening any PR."
+              echo "changes_made=false" >> "$GITHUB_ENV"
+          fi
+      - name: Create Pull Request
+        if: env.changes_made == 'true'
+        uses: peter-evans/create-pull-request@v7
+        env:
+          NIGHTLY_DATE: ${{ env.nightly_date }}
+        with:
+          author: Update Nightly Rustc Bot <[email protected]>
+          committer: Update Nightly Rustc Bot <[email protected]>
+          title: Automated update to rustc (to nightly-$NIGHTLY_DATE)
+          body: |
+            Automated update to Rust nightly by [create-pull-request](https://github.com/peter-evans/create-pull-request) GitHub action
+          commit-message: Automated update to Rust nightly-$NIGHTLY_DATE
+          branch: create-pull-request/automated-nightly-update