Match the Rails name for sqlite in `$DB`

[elia]

Summary

We traditionally used sqlite and had code to convert it to sqlite3 when using it as a rails new --database=… option. Moving to use the same naming simplifies the setup. Sandbox and solidus_core script should already be compatible with the rails-compatible name.

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[elia]

Reopening from a branch in the same repo so the publishing CI step will work, see #66.