sagemathgh-36694: CI conda: On `pull_request`, only run 1 macOS job and 1 Linux job
    
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as discussed in
sagemath#36678 (comment),
sagemath#36616 (comment)

On pushes to a tag, all 3 Linux and 3 macOS jobs are still run, as
demonstrated in https://github.com/mkoeppe/sage/actions/runs/6829619271

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After merging, the branch protection rule https://github.com/sagemath/sa
ge/settings/branch_protection_rules/33965567 will need to be updated. It
controls what workflows show as "Required" in the PR Checks.

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URL: sagemath#36694
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Tobias Diez

Author: Release Manager

.github/workflows/ci-conda.yml

@@ -22,11 +22,26 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu, macos]
-        python: ['3.9', '3.10', '3.11']
+        # On pushes to tags or branches, test the whole matrix.
+        os: >-
+          ${{ github.event_name == 'pull_request'
+                && fromJson('["ubuntu"]')
+                || fromJson('["ubuntu", "macos"]') }}
+        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", "python": "3.10", "conda-env": "environment"}]')
+                || fromJson('[]') }}
 
     steps:
       - uses: actions/checkout@v4