Add new workflow step to validate previous release

[arboleya]

The idea is that for every new release involving a fuel-core upgrade, we run our e2e tests against the test/dev networks:

1. Once for the previous release (current, soon to be previous)
2. Once for PR in question (next release)

This should provide answers to these questions before releasing a fuel-core upgrade:

1. Is the previous release still working with dev and test nets?
2. Is the new release working with dev and test nets?

[arboleya]

I think this covers recent discussions. Otherwise, feel free to edit the description.

cc @Torres-ssf @nedsalk @petertonysmith94 @FuelLabs/sdk-ts