fix(studio-bridge): use humantime format for container retention cut-off

snok/container-retention-policy v3.0.0 uses Rust humantime crate,
not the old "30 days ago UTC" format.

Author: Quenty

.github/workflows/studio-linux-docker-build.yml

@@ -104,5 +104,5 @@ jobs:
           account: quenty
           token: ${{ secrets.GITHUB_TOKEN }}
           image-names: nevermore-studio-linux
-          cut-off: 30 days ago UTC
+          cut-off: 30d
           keep-n-most-recent: 5