ci: make sure to run tezt:build-long only on dev branches

Context

tezt:build-long should run only on dev branches, but not on master and release branches.

This MR sets the job to run in the correct pipeline

Manually testing the MR

I've added a test commit on top of this MR to check if effectively the test is not run on master or release pipelines.

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