Bump vega-agent action to v0.4.3 (sends upload fileHash; staging now requires it)

The pinned @53f705c predated the upload-url fileHash requirement, so staging
returned 400 on the presigned-upload request. v0.4.3 sends fileHash.

Author: jasonodoom

.github/workflows/attest.yml

@@ -10,7 +10,7 @@ jobs:
   attest:
     runs-on: ubuntu-latest
     steps:
-      - uses: Ad-Astra-Computing/vega-agent/agent@53f705ce2741d70e41122c1ddc11efe088fea108
+      - uses: Ad-Astra-Computing/vega-agent/agent@v0.4.3
         with:
           installable: "github:Ad-Astra-Computing/vega-test-a#figlet"
           control-plane: https://vega-cache-staging.ad-astra-computing-inc.workers.dev