ci: set POETRY=poetry in release job so make targets use Poetry's managed venv

The pre-tag gates (run-pre-commit/audit/checkmigrations) called tools from
venv/bin, but without POETRY set the Makefile takes its ./venv branch while
Poetry installs into its own managed virtualenv, so venv/bin/pre-commit never
existed and the gate failed with exit 127. python.yaml already sets this
workflow-wide; mirror it on the release job.

Author: yyyyyyyan

.github/workflows/release.yml

@@ -29,6 +29,8 @@ jobs:
   release:
     runs-on: ubuntu-latest
     environment: release
+    env:
+      POETRY: poetry
     steps:
       - name: Restrict to release managers
         if: github.actor != 'yyyyyyyan' && github.actor != 'maxbube'