[skip ci] Fix nightly workflow dispatch

Closes GH-16662

[skip ci] Fix nightly job name for 8.1

[skip ci] Move nightly to workflow_call

This PR removes most of the logic of nightly_matrix.php by converting
nightly.yml to a workflow_call, and invoking it multiple times for each bran

[skip ci] Move nightly to workflow_call

This PR removes most of the logic of nightly_matrix.php by converting
nightly.yml to a workflow_call, and invoking it multiple times for each branch
in a new root.yml workflow. The naming of the files is intentionally kept to
make the diff readable. They may still be renamed afterward.

Closes GH-16642

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