Pass the DATABASE_URL to command spawned for sqlx-macros

[CosmicHorrorDev]

Currently cargo sqlx prepare doesn't pass the database url value to the child command spawned for sqlx-macros.

$ DATABASE_URL="..." cargo sqlx prepare

works as expected because sqlx-macros will read the value from the env var which is inherited, but

$ cargo sqlx prepare --database-url "..."

will fail in some way (An existing sqlx-data.json makes it not fail, but causes it to find no queries. No existing sqlx-data.json will lead to a error: failed to find data for query) because the database url isn't being passed to the child process.

It's also worth noting that these changes appear to obey precedence correctly too where

$ DATABASE_URL="A" cargo sqlx prepare --database-url "B"

will lead to "B" being used which should be right. I wasn't certain that the old env var would be overridden until I tested it, but it follows what I would have assumed.