[macOS CI] Delete big SDKs to avoid exhausting disk space

Lately we've seen frequent failures on macOS GitHub Action runs due to
disk space issues. Poking with du(1) revealed that
/Library/Developer/CoreSimulator/Caches/dyld was growing to be multiple
gigbytes.

Deleting unused stuff is a known workaround to space issues.

https://github.com/actions/runner-images/issues/2840#issuecomment-790492173

.github/workflows/macos.yml

@@ -80,6 +80,14 @@ jobs:
           sudo sysctl -w kern.coredump=1
           sudo chmod -R +rwx /cores/
 
+      - name: Delete unused SDKs
+        # To free up disk space to not run out during the run
+        run: |
+          sudo rm -rf ~/.dotnet
+          sudo rm -rf /Library/Android
+          sudo rm -rf /Library/Developer/CoreSimulator
+        continue-on-error: true
+
       - name: Run configure
         run: ../src/configure -C --disable-install-doc ${ruby_configure_args}