Try to kill XProtect before running Mac CI (#30779)

This is an attempt to fix errors on the Mac CI when running `hdiutil`
that look like this:

```
Run python3 ./mach package --release
hdiutil: create failed - Resource busy
Creating Servo.app
Copying files
Swapping prefs
Finding dylibs and relinking
Adding version to Credits.rtf
Creating dmg
Packaging MacOS dmg exited with return value 1
Error: Process completed with exit code 1.
```

This approach was taken from
https://github.com/actions/runner-images/issues/7522.

.github/workflows/mac.yml

@@ -56,6 +56,13 @@ jobs:
     name: Mac Build
     runs-on: macos-13
     steps:
+      # XProtect can cause random failures if it decides that the DMG we create
+      # during the packaging phase is malware.
+      # TODO(mrobinson): Is there a way we can do things in a less suspicious way so
+      # we don't have to kill this service?
+      - name: Kill XProtectBehaviorService
+        run: |
+          echo Killing XProtect.; sudo pkill -9 XProtect >/dev/null || true;
       - uses: actions/checkout@v3
         if: github.event_name != 'pull_request_target'
         with: