ci: Add workflow to upload nightly wheels to Anaconda Cloud

* On a schedule that follows _after_ the schedule job for the packaging
  tests workflow, use the gh command line API to find the latest run of
  the packaging tests workflow. Filtering on 'schedule' events to find
  the latest schedule run, extract the run id, and then use that to
  download all of the 'awkward-wheel' and 'awkward-cpp-wheels-*'
  artifacts from the workflow.
* Use the scientific-python/upload-nightly-action GitHub Action to then
  upload these wheels to the scikit-hep-nightly-wheels Anaconda Cloud
  organization registry.
   - c.f. https://github.com/scientific-python/upload-nightly-action
   - c.f. https://anaconda.org/scikit-hep-nightly-wheels

.github/workflows/upload_nightly_wheels.yml

@@ -0,0 +1,65 @@
+name: Upload nightly wheels to Anaconda Cloud
+
+on:
+  # Run daily at 2:34 UTC to upload nightly wheels to Anaconda Cloud
+  schedule:
+    - cron: '34 2 * * *'
+  # Run on demand with workflow dispatch
+  workflow_dispatch:
+
+permissions:
+  actions: read
+
+jobs:
+  upload_nightly_wheels:
+    name: Upload nightly wheels to Anaconda Cloud
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        # The login shell is necessary for the setup-micromamba setup
+        # to work in subsequent jobs.
+        # https://github.com/mamba-org/setup-micromamba#about-login-shells
+        shell: bash -e -l {0}
+    if: github.repository_owner == 'scikit-hep'
+
+    steps:
+      # https://github.com/actions/download-artifact/issues/3#issuecomment-1017141067
+      - name: Download wheel artifacts from last build on 'main'
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          PROJECT_REPO="scikit-hep/awkward"
+          BRANCH="main"
+          WORKFLOW_NAME="packaging-test.yml"
+          ARTIFACT_PATTERN="awkward*wheel*"  # awkward-wheel and awkward-cpp-wheels-*
+
+          gh run --repo "${PROJECT_REPO}" \
+             list --branch "${BRANCH}" \
+                  --workflow "${WORKFLOW_NAME}" \
+                  --json event,status,conclusion,databaseId > runs.json
+          RUN_ID=$(
+            jq --compact-output \
+              '[
+                .[] |
+                # Filter on "schedule" events to main (nightly build) ...
+                select(.event == "schedule") |
+                # that have completed successfully ...
+                select(.status == "completed" and .conclusion == "success")
+               ] |
+              # and get ID of latest build of wheels.
+              sort_by(.databaseId) | reverse | .[0].databaseId' runs.json
+          )
+          gh run --repo "${PROJECT_REPO}" view "${RUN_ID}"
+          gh run --repo "${PROJECT_REPO}" \
+             download "${RUN_ID}" --pattern "${ARTIFACT_PATTERN}"
+
+          mkdir dist
+          mv ${ARTIFACT_PATTERN}/*.whl dist/
+          ls -l dist/
+
+      - name: Upload wheels to Anaconda Cloud as nightlies
+        uses: scientific-python/upload-nightly-action@6e9304f7a3a5501c6f98351537493ec898728299 # 0.3.0
+        with:
+          artifacts_path: dist
+          anaconda_nightly_upload_organization: scikit-hep-nightly-wheels
+          anaconda_nightly_upload_token: ${{ secrets.ANACONDA_ORG_UPLOAD_TOKEN }}