Fix: adding DB_URI var when building docs in release (#151)

## Description
The .env no longer exists in the repo, so we must manually set a value
for the required DB_URI

## Additional Notes
Example build failure
https://github.com/CDCgov/RecordLinker/actions/runs/12319787147/job/34387640857

.github/workflows/release.yml

@@ -83,6 +83,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install '.[dev]'
+          export DB_URI="sqlite:///:memory:"
           export INITIAL_ALGORITHMS=""
           export VERSION=${{ env.NEXT_TAG }}
           ./scripts/build_docs.sh _site