fetchs3: allow to name the derivation output (#39823)

* fetchs3: add configurable name

Change the default from "foo" to the basename of the s3 URL and make it
configurable.

* fetchs3: fix error on missing credentials.session_token

The session token should default to null instead of failing

* fetchs3: make use of the region argument

Set it to null if you don't want to use it

* fetchs3: prefer local build

Fetcher-types spend more time on network than CPU