DATABASE_URL must be set for `sqlx mig add`

[jplatte]

Not a big deal since I usually have DATABASE_URL in .env anyways, but I just noticed in a new project before creating .env that sqlx mig add init failed with

error: The DATABASE_URL environment variable must be set

Which there doesn't seem to be a good reason for.

[abonander]

Yeah, it's purely because we're requiring it unconditionally: https://github.com/launchbadge/sqlx/blob/master/sqlx-cli/src/lib.rs#L18-L22

clap has support for getting values from the environment, so we could move this to the Opt definitions, e.g.:

#[derive(clap::Clap)]
enum MigrateOpt {
    Add {
        #[clap(long, env)]
        database_url: String,
        // ...
    }
}

It'd just be a bit annoying to have to do this for every command.

The error message that Clap generates if you omit DATABASE_URL isn't really helpful though as it defaults to suggesting it as --database-url=... and doesn't mention environment variables at all except in the output of --help.

[xpe]

The commit made in this issue (0e51272) led to a regression (#1431) where a .env file isn't read in time for clap argument parsing.