debug out-of-disk-space errors on macOS CI

See:

* https://github.com/ruby/ruby/actions/runs/10604551010
* https://github.com/ruby/ruby/actions/runs/10604465444/job/29391187275#step:14:4
* https://github.com/ruby/ruby/actions/runs/10604465444/job/29391187275

.github/workflows/macos.yml

@@ -61,6 +61,9 @@ jobs:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
 
+      - name: show disk usage
+        run: df -h
+
       - name: Install libraries
         uses: ./.github/actions/setup/macos
 
@@ -74,6 +77,9 @@ jobs:
           # Set fetch-depth: 0 so that Launchable can receive commits information.
           fetch-depth: 10
 
+      - name: show disk usage
+        run: df -h
+
       - name: make sure that kern.coredump=1
         run: |
           sysctl -n kern.coredump
@@ -83,11 +89,20 @@ jobs:
       - name: Run configure
         run: ../src/configure -C --disable-install-doc ${ruby_configure_args}
 
+      - name: show disk usage
+        run: df -h
+
       - run: make prepare-gems
         if: ${{ matrix.test_task == 'test-bundled-gems' }}
 
+      - name: show disk usage
+        run: df -h
+
       - run: make
 
+      - name: show disk usage
+        run: df -h
+
       - name: Set test options for skipped tests
         run: |
           set -x
@@ -105,6 +120,9 @@ jobs:
           srcdir: src
         continue-on-error: true
 
+      - name: show disk usage
+        run: df -h
+
       - name: Set extra test options
         run: |
           echo "TESTS=$TESTS ${{ matrix.test_opts }}" >> $GITHUB_ENV