only use build isolation when needed

.github/workflows/tests.yml

@@ -158,8 +158,15 @@ jobs:
         if: ${{ matrix.os == 'windows-2019' }}
 
       - name: Build and install Shapely
+        # for the numpy nightly build, we temporarily need to build without
+        # the standard build isolation based on build-system.requires, to
+        # ensure we build against numpy nightly (numpy 2.0 compat)
         run: |
-          pip install -e . --no-build-isolation
+          if [ -z "${{ matrix.numpy }}" ]; then
+            pip install -e . --no-build-isolation
+          else
+            pip install -e .
+          fi
 
       - name: Overview of the Python environment (pip list)
         run: pip list