Update release workflows

.github/workflows/pypi-release.yaml

@@ -6,11 +6,19 @@ on:
   push:
     tags:
       - "v*"
+  pull_request:
+    types: [opened, reopened, synchronize, labeled]
+  workflow_dispatch:
 
 jobs:
   build-artifacts:
     runs-on: ubuntu-latest
-    if: github.repository == 'pydata/xarray'
+    if: ${{ github.repository == 'pydata/xarray' && (
+         (contains(github.event.pull_request.labels.*.name, 'Release') && github.event_name == 'pull_request') ||
+         github.event_name == 'release' ||
+         github.event_name == 'workflow_dispatch' ||
+         startsWith(github.ref, 'refs/tags/v')
+       ) }}
     steps:
       - uses: actions/checkout@v4
         with:
@@ -64,7 +72,6 @@ jobs:
           ls -ltrh dist
 
       - name: Verify the built dist/wheel is valid
-        if: github.event_name == 'push'
         run: |
           python -m pip install --upgrade pip
           python -m pip install dist/xarray*.whl

HOW_TO_RELEASE.md

@@ -48,6 +48,8 @@ upstream        https://github.com/pydata/xarray (push)
       release.
 5.  Open a PR with the release summary and whatsnew changes; in particular the
     release headline should get feedback from the team on what's important to include.
+    Apply the `Release` label to the PR to trigger a test build action.
+
 6.  After merging, again ensure your main branch is synced to upstream:
     ```sh
     git pull upstream main