Update GHA for uploading artifacts (#19)

Author: jjngx

.github/actions/sign/action.yml

@@ -35,7 +35,7 @@ runs:
       shell: bash
 
     - name: Store assertion document
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@330a01c490aca151604b8cf639adc76d48f6c5d4 # v5.0.0
       with:
         name: ${{ steps.sign.outputs.assertiontar }}
         path: ${{ steps.sign.outputs.assertiontar }}