.github/workflows/ci-conda.yml: On pull_request, only run 1 Linux job

Author: Matthias Koeppe

.github/workflows/ci-conda.yml

@@ -24,17 +24,25 @@ jobs:
       fail-fast: ${{ github.event_name == 'pull_request' }}
       max-parallel: ${{ github.event_name == 'pull_request' && 2 || 6 }}
       matrix:
+        # On pushes to tags or branches, test the whole matrix.
         os: >-
           ${{ github.event_name == 'pull_request'
                 && fromJson('["ubuntu-latest"]')
                 || fromJson('["ubuntu-latest", "macos-latest"]') }}
-        python: ['3.9', '3.10', '3.11']
+        python: >-
+          ${{ github.event_name == 'pull_request'
+                && fromJson('["3.9"]')
+                || fromJson('["3.9", "3.10", "3.11"]') }}
         # Optional environment is disabled for now as its not yet working
         # environment: [environment, environment-optional]
         conda-env: [environment]
+        # On pull requests, only test two jobs:
+        # Ubuntu with Python 3.9, macOS with Python 3.10.
+        # Build & Test currently uses Python 3.11 (on ubuntu-focal).
+        # Together, they cover the supported minor Python versions.
         include: >-
           ${{ github.event_name == 'pull_request'
-                && fromJson('[{"os": "macos-latest", "python": "3.11", "conda-env": "environment"}]')
+                && fromJson('[{"os": "macos-latest", "python": "3.10", "conda-env": "environment"}]')
                 || fromJson('[]') }}
 
     steps: