SEP-1001: Fix Coverage (main) workflow by setting POETRY env var (#552)

Add a workflow-level `env: POETRY: poetry` declaration to
`.github/workflows/coverage-main.yml`, mirroring
`.github/workflows/python.yaml` (lines 13-14).

The `Coverage (main)` workflow has been failing on every push to `main`
because `make test` exits with code 127 — without the `POETRY` env var,
the `Makefile`'s `ifdef POETRY` branch is skipped and `VENV` falls
through to its default value, so `${VENV_BIN}/pytest` resolves to a
non-existent path. With the env var set, `VENV` is resolved via `poetry
env info --path`, pointing at the Poetry-managed virtualenv that `poetry
install --with syncmysql` populates.

Author: yyyyyyyan

.github/workflows/coverage-main.yml

@@ -13,6 +13,9 @@ on:
 permissions:
   contents: read
 
+env:
+  POETRY: poetry
+
 jobs:
   store-coverage:
     runs-on: ubuntu-latest