ci: drop the git-crypt .env before running flask

.env is committed git-crypt encrypted. Without the key the checked out
file is the raw encrypted blob, and python-dotenv fails reading it as
UTF-8 with "invalid start byte". The migrations job already provides
DATABASE_URL, SECRET_KEY and JWT_SECRET_KEY through env:, so removing
the file is enough. Previously masked by the job dying earlier on a
missing mysql client.

.gitea/workflows/ci.yml

@@ -115,6 +115,13 @@ jobs:
           python3 -m venv .venv
           .venv/bin/pip install --upgrade pip
           .venv/bin/pip install -r requirements.txt
+      - name: Drop the git-crypt .env
+        # .env is committed git-crypt encrypted. CI has no key, so the checked
+        # out file is the raw encrypted blob and python-dotenv dies reading it
+        # as UTF-8. This job supplies DATABASE_URL, SECRET_KEY and
+        # JWT_SECRET_KEY through env: already, so the file is not needed.
+        run: rm -f .env
+
       - name: Force utf8mb4 on the CI database
         # Uses pymysql, already a runtime dependency, rather than the mysql CLI,
         # which is not present in every runner image.