Post message to run e2e only when job is available

What does this MR do and why?

This is a follow-up of !127359 (merged).

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This MR ensures the trigger-omnibus-and-follow-up-e2e job is available when the message for running it is posted.

We create a separate job for the generate-message-to-run-e2e-pipeline.rb script that needs:

1. e2e-test-pipeline-generate => This populates the QA_TESTS env variable that the generate-message-to-run-e2e-pipeline.rb script uses
2. trigger-omnibus-env => Needed by trigger-omnibus which the trigger-omnibus-and-follow-up-e2e extends.
3. build-assets-image => Needed by trigger-omnibus which the trigger-omnibus-and-follow-up-e2e extends.

Also adds text mentioning why the job is manual and that the e2e test jobs are allowed to fail due to flakiness.

Tested on this temporary MR: !127543 (closed)

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