workflows: fix clippy failing to read database_url

SQLx compile-time-checks need DATABASE_URL to be set to use query
macros. Since it doesn't actually connect to the database, an in-memory
SQLite should be fine.

.github/workflows/lint.yml

@@ -8,6 +8,8 @@ jobs:
   clippy:
     name: Clippy
     runs-on: ubuntu-latest
+    env:
+      DATABASE_URL: "sqlite::memory:"
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4