Trigger based on label

Author: maxrjones

.github/workflows/downstream.yml

@@ -2,6 +2,8 @@ name: Downstream
 
 on:
   workflow_dispatch:
+  pull_request:
+    types: [labeled]
 
 permissions:
   contents: read
@@ -13,6 +15,7 @@ concurrency:
 jobs:
   xarray:
     name: Xarray zarr backend tests
+    if: github.event_name == 'workflow_dispatch' || github.event.label.name == 'run-downstream'
     runs-on: ubuntu-latest
     steps:
       - name: Check out zarr-python
@@ -55,6 +58,7 @@ jobs:
 
   numcodecs:
     name: numcodecs zarr3 codec tests
+    if: github.event_name == 'workflow_dispatch' || github.event.label.name == 'run-downstream'
     runs-on: ubuntu-latest
     steps:
       - name: Check out zarr-python

.github/workflows/prepare_release.yml

@@ -73,4 +73,5 @@ jobs:
             - [ ] Downstream tests pass (see [downstream workflow](https://github.com/zarr-developers/zarr-python/actions/workflows/downstream.yml))
             - [ ] Merge this PR, then [draft a GitHub Release](https://github.com/zarr-developers/zarr-python/releases/new) targeting `${{ inputs.target_branch }}` with tag `v${{ inputs.version }}`
           commit-message: "chore: build changelog for v${{ inputs.version }}"
+          labels: run-downstream
           delete-branch: true