fix(ci): add crates-io environment for Trusted Publishing (#504)

Trusted Publishing on crates.io requires the workflow to specify an
environment that matches the one configured in the trusted publisher.

The environment 'crates-io' must be created in GitHub repo settings.

Author: joshrotenberg

.github/workflows/release-plz.yml

@@ -18,6 +18,7 @@ jobs:
   release-plz:
     name: Release-plz
     runs-on: ubuntu-latest
+    environment: crates-io
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4