[upstream_utils] Document how to update thirdparty libraries (#4253)

Also, add a CI job to ensure the sources in the repo are consistent with
the update scripts.

.github/workflows/upstream-utils.yml

@@ -0,0 +1,53 @@
+name: Upstream utils
+
+on:
+  pull_request:
+  push:
+    branches-ignore:
+      - main
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  update:
+    name: "Update"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Fetch all history and metadata
+        run: |
+          git fetch --prune --unshallow
+          git checkout -b pr
+          git branch -f main origin/main
+      - name: Set up Python 3.9
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+      - name: Configure committer identity
+        run: |
+          git config --global user.email "you@example.com"
+          git config --global user.name "Your Name"
+      - name: Run update_drake.py
+        run: |
+          cd upstream_utils
+          ./update_drake.py
+      - name: Run update_eigen.py
+        run: |
+          cd upstream_utils
+          ./update_eigen.py
+      - name: Run update_libuv.py
+        run: |
+          cd upstream_utils
+          ./update_libuv.py
+      - name: Run update_llvm.py
+        run: |
+          cd upstream_utils
+          ./update_llvm.py
+      - name: Run update_stack_walker.py
+        run: |
+          cd upstream_utils
+          ./update_stack_walker.py
+      - name: Check output
+        run: git --no-pager diff --exit-code HEAD