Fix script and CI: snapshot_alpha_and_link.sh

What

This MR fixes scripts/link_protocol.sh, which was broken since the definition of protocols were moved from manifest/main.ml to manifest/product_octez.ml.

It also fixes the test in the CI, which should have detected this issue but did not.

Why

For the greater good.

How

The CI script did not detect the failure because it didn't exit if a step of the script script:snapshot_alpha_and_link.sh failed; instead it just continued. Adding set -e fixes this.

link_protocol.sh was fixed by changing the file where to look for the definition of protocol Alpha to add the new protocol.

Manually testing the MR

https://gitlab.com/tezos/tezos/-/pipelines/1308528898 is a pipeline that was run with only the first commit of this MR, which causes the CI to correctly detect the issue. So this pipeline fails (see job https://gitlab.com/tezos/tezos/-/jobs/6957802079).

https://gitlab.com/tezos/tezos/-/pipelines/1308618679 is a pipeline with both fixes: the fix to detect failures, and the fix to the failure. The job now succeeds: https://gitlab.com/tezos/tezos/-/jobs/6958355664

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR