Add a workflow file to update the nightly version every month (#118)

Part of https://github.com/serai-dex/serai/issues/116

.github/nightly-version

@@ -1 +1 @@
-nightly-2022-09-01
+nightly-2022-08-01

.github/workflows/monthly-nightly-update.yml

@@ -0,0 +1,57 @@
+name: Monthly Nightly Update
+
+on:
+  push:
+  schedule:
+    - cron: "0 0 1 * *"
+
+jobs:
+  update:
+    name: Update nightly
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          submodules: "recursive"
+
+      - name: Write nightly version
+        run: echo $(date +"nightly-%Y-%m"-01) > .github/nightly-version
+
+      - name: Create the commit
+        id: commit
+        run: |
+          git config user.name "GitHub Actions"
+          git config user.email "<>"
+
+          git checkout -b $(date +"nightly-%Y-%m")
+
+          git add .github/nightly-version
+          git commit -m "Update nightly"
+          git push -u origin $(date +"nightly-%Y-%m")
+
+          echo "::set-output name=commit::$(git rev-parse HEAD)"
+
+      - name: Pull Request
+        uses: actions/github-script@v6
+        with:
+          script: |
+            const { repo, owner } = context.repo;
+
+            const result = await github.rest.pulls.create({
+              title: (new Date()).toLocaleString(
+                false,
+                { month: "long", year: "numeric" }
+              ) + " - Rust Nightly Update",
+              owner,
+              repo,
+              head: "${{ steps.commit.outputs.commit }}",
+              base: "develop",
+              body: "PR auto-generated by a GitHub workflow."
+            });
+
+            github.rest.issues.addLabels({
+              owner,
+              repo,
+              issue_number: result.data.number,
+              labels: ["improvement"]
+            });