Do not require db url for prepare

[tamasfe]

Allow per-crate DATABASE_URL in .env files for workspaces to be used alongside cargo sqlx prepare. Closes #2667.

Instead of enforcing the database-url argument by clap, I made it optional and instead throw an error later if for some reason it has to be provided (e.g. migrations).

I also had to remove the eager db checks for the prepare commands, the queries in cargo check will fail anyway.

[abonander]

Closing due to inactivity, feel free to reopen but please address the review comment.

It's worth checking eagerly that the database exists because it can provide quicker feedback in the case that some dependencies need to be built.