ci: disable nightly build in forked repos

The forked repos nightly build will waste a myriad of CPU resources.

.github/workflows/release-ci.yml

@@ -33,7 +33,7 @@ jobs:
           merge-multiple: true
 
       - name: Create Nightly release
-        if: ${{ github.ref == 'refs/heads/master' }}
+        if: ${{ github.repository == 'rime/librime' && github.ref == 'refs/heads/master' }}
         uses: 'marvinpinto/action-automatic-releases@latest'
         with:
           repo_token: ${{ secrets.GITHUB_TOKEN }}