Permission denied when caching `macports`

Installing macports is a time-consuming part in my CI, so I guess backing up /opt/local will help:

- name: Cache MacPorts
  id: cache-macports
  uses: actions/cache@v2
  with:
    path: /opt/local/
    key: ${{ runner.os }}-macports-${{ hashFiles('macos_build.txt') }}

However, since #133 didn't accept using sudo tar, I got a long error log all about permissons:

Run actions/cache@v2
Received 138412032 of 255723896 (54.1%), 132.0 MBs/sec
Received 255723896 of 255723896 (100.0%), 105.3 MBs/sec
Cache Size: ~244 MB (255723896 B)
/usr/local/bin/gtar --use-compress-program zstd -d -xf /Users/runner/work/_temp/68ab3e83-d089-4f96-b645-0834881ca4e5/cache.tzst -P -C /Users/runner/work/pacaptr/pacaptr --delay-directory-restore
/usr/local/bin/gtar: ../../../../../opt/local: Cannot mkdir: Permission denied
/usr/local/bin/gtar: ../../../../../opt/local: Cannot mkdir: Permission denied

Is it possible to introduce some sort of sudo, or is there any workaround?

Thanks in advance!