Remove redundant tasks in CI pipelines

The pipeline organization is not efficient.

For instance:

• tests are run right before a MR is merged and just after it's merged into its destination branch.
• tests are run multiple on a same git node every time it's duplicated (new empty branch, new tag...) -- yet there is the issue the original node could old, and external dependencies may have changed since then when we fork it.
• pip installation of S1Tiling is done once for tests, once for documentation generation, once for generating whee... We should rely more on artefacts