CI: test on all runners when the build logic changes

Co-Authored-By: Ivan Pozdeev <vano@mail.mipt.ru>

.github/actions/build-python/action.yml

@@ -1,8 +1,7 @@
 name: Build a Python version
 description: >
   Install python-build's system dependencies for the current runner's OS, build
-  the requested version from source with `pyenv install`, and check that the
-  interpreter that comes out is the one that was asked for.
+  and check the requested version.
 inputs:
   python-version:
     description: A version as accepted by `pyenv install`.

.github/workflows/build.yml

@@ -8,20 +8,45 @@ permissions:
   contents: read
 
 jobs:
+  discover_build_logic_change:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-slim
+    env:
+      FULL_OS_MATRIX: '[
+        "macos-14",
+        "macos-15",
+        "macos-15-intel",
+        "macos-26",
+        "macos-26-intel",
+        "ubuntu-22.04",
+        "ubuntu-22.04-arm",
+        "ubuntu-24.04",
+        "ubuntu-24.04-arm"
+      ]'
+    outputs:
+      os: ${{ steps.detect.outputs.os }}
+    steps:
+      - uses: actions/checkout@v7
+      - run: git fetch origin "$GITHUB_BASE_REF"
+      - id: detect
+        shell: bash
+        run: |
+          if [[ -n $(git diff --name-only "origin/$GITHUB_BASE_REF" -- \
+                       plugins/python-build/bin/python-build \
+                       .github/actions/build-python/action.yml \
+                       .github/workflows/build.yml) ]]
+          then
+            printf "%s\n" "os<<!" "$FULL_OS_MATRIX" "!" >> "$GITHUB_OUTPUT"
+          fi
+
   build:
+    needs: discover_build_logic_change
+    # Run when the detection job succeeded *or was skipped* -- but not failed
+    if: ${{ !cancelled() && !failure() }}
     strategy:
       fail-fast: false
       matrix:
-        os:
-          - macos-14
-          - macos-15
-          - macos-15-intel
-          - macos-26
-          - macos-26-intel
-          - ubuntu-22.04
-          - ubuntu-22.04-arm
-          - ubuntu-24.04
-          - ubuntu-24.04-arm
+        os: ${{ fromJson(needs.discover_build_logic_change.outputs.os || '["macos-latest","ubuntu-latest"]') }}
         python-version:
           - "3.10"
           - "3.11"