[macros] improve handling of `SQLX_OFFLINE`

[abonander]

Currently, the macros expect SQLX_OFFLINE to equal "true" (case insensitive) to force offline mode. This is fine, but environment variables that can switch something on or off typically also accept 0 as false and 1 as true.

This should be a relatively easy fix, the code is here: https://github.com/launchbadge/sqlx/blob/master/sqlx-macros/src/query/mod.rs#L40

I would also prefer to use str::eq_ascii_ignore_case here to save an allocation.