gh-actions: update 'artifact' actions

It was failing with:

Error: This request has been automatically failed because it uses a
deprecated version of `actions/upload-artifact: v2`. Learn more:
https://github.blog/changelog/2024-02-13-deprecation-notice-v1-and-v2-of-the-artifact-actions/

.github/workflows/release.yml

@@ -37,15 +37,15 @@ jobs:
         if: matrix.job.os != 'darwin'
         run: |
           make upx
-      - uses: actions/upload-artifact@v2
+      - uses: actions/upload-artifact@v4
         with:
           path: dist/
           name: dist
   release:
     needs: build
     runs-on: ubuntu-22.04
     steps:
-      - uses: actions/download-artifact@v2
+      - uses: actions/download-artifact@v4
         with:
           path: dist/
           name: dist