fix(macros): smarter `.env` loading, caching, and invalidation (#4053)

* fix(macros): smarter `.env` loading, caching, and invalidation

* feat(mysql): test `.env` loading in CI

* feat(postgres): test `.env` loading in CI

* feat(macros): allow `DATABASE_URL` to be empty

* fix(examples/postgres): make `cargo-sqlx` executable

* fix(examples/postgres): `cargo sqlx` invocation

* feat(examples/postgres): check offline prepare on more examples

* fix(examples/postgres): the name of this step

* fix(cli): don't suppress error from `dotenv()`

* fix(ci/examples/postgres): don't use heredoc in this step

* fix(ci/examples/postgres): multi-tenant

* fix(ci/examples/sqlite): test `.env` loading

* chore: add CHANGELOG entry