rework branch-is-up-to-date-with-target-pre job

See sa4/audio/ivas-basop#2033. This implements the following logic in the branch-is-up-to-date-with-target-pre job:

flowchart TD
    A{Branch is up-to-date with target?}
    B(exit 1 - failure. Artifacts: None)
    C(Get target branch current commit SHA and store in file for artifacts)
    F(Get float_ref branch - which can be ivas-float-update or not - from CI_MERGE_REQUEST_SOURCE_BRANCH_NAME)
    G(Get float_ref current commit SHA and store in file for artifacts)
    H(Get ivas-float-update current commit SHA and store in file for artifacts)
    I(exit 0 - success. Artifacts: MERGE_TARGET_COMMIT_FILE, FLOAT_REF_COMMIT_FILE, MERGE_SOURCE_FLOAT_REF_COMMIT_FILE)
    
    A -->|No| B
    A -->|Yes| C
    C --> F
    F --> G
    G --> H
    H --> I

Git refs are passed to subsequent jobs where they are needed, i.e. the check-be/regression jobs + the *-pytest-on-merge-request ones. This ensures that over the pipeline runtime, we always use the same main commit, the same ivas-float-update commit and the same xxx_ref_blabla commit (if porting branch).