.github: Add an automatic package updates workflow

It will run on every Monday and will try to update packages listed in
the package listing. This can be used to gradually add more and more
packages under the automation.

sdk_container/src/third_party/portage-stable/.github/workflows/packages-list

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+# Please keep the list sorted!

sdk_container/src/third_party/portage-stable/.github/workflows/update-packages-from-list.yml

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+name: Keep updated packages updated
+on:
+  schedule:
+    - cron:  '0 7 * * 1'
+  workflow_dispatch:
+
+jobs:
+  keep-packages-updated:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout portage-stable
+        uses: actions/checkout@v3
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          path: ./portage-stable
+      - name: Checkout gentoo
+        uses: actions/checkout@v2
+        with:
+          repository: gentoo/gentoo
+          path: gentoo
+          fetch-depth: 0
+      - name: Checkout build scripts
+        uses: actions/checkout@v2
+        with:
+          repository: flatcar/flatcar-build-scripts
+          ref: krnowak/stuff
+          path: flatcar-build-scripts
+      - name: Update listed packages
+        id: update-listed-packages
+        run: |
+          git config --global user.name "Flatcar Buildbot"
+          git config --global user.email "buildbot@flatcar-linux.org"
+          old_head=$(git -C portage-stable rev-parse HEAD)
+          cd portage-stable
+          while read -r package; do
+              if [[ ! -d "${package}" ]]; then
+                  # If this happens, it means that the package was moved to overlay
+                  # or dropped, the list ought to be updated.
+                  echo "::warning title=${package}::Nonexistent package"
+                  continue
+              fi
+              GENTOO_REPO=../gentoo ../flatcar-build-scripts/sync-with-gentoo "${package}"
+          done < <(grep '^[^#]' .github/workflows/packages-list)
+          cd ..
+          new_head=$(git -C portage-stable rev-parse HEAD)
+          updated=0
+          count=0
+          if [[ "${new_head}" != "${old_head}" ]]; then
+              updated=1
+              count=$(git -C portage-stable rev-list --count "${old_head}..${new_head}")
+          fi
+          echo ::set-output "name=UPDATED::${updated}"
+          echo ::set-output "name=COUNT::${count}"
+      - name: Create pull request for main branch
+        uses: peter-evans/create-pull-request@v4
+        if: steps.update-listed-packages.outputs.UPDATED == 1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          path: portage-stable
+          branch: buildbot/automatic-package-updates
+          delete-branch: true
+          base: main
+          title: Weekly package updates
+          body: Updated ${{steps.update-listed-packages.outputs.COUNT }} package(s).
+          labels: main