Fallback to `READ_ONLY_REPLICA_URL` for db_dump
Fallback to `READ_ONLY_REPLICA_URL` for db_dump

In production, the scheduler was configured to run:

```
./target/release/enqueue-job dump_db $READ_ONLY_REPLICA
```

instead of:

```
./target/release/enqueue-job dump_db $READ_ONLY_REPLICA_URL
```

Because of the typo in the variable name, the dump_db job has been
running against the primary database.  This patch modifies the fallback
to ensure the primary database is only used if explicitly provided.

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io