ci: add names to compatibility matrix steps for readability

[MGPOCKY]

Problem

In the compatibility test workflows, the set-matrix step is a long unnamed run block, so the Actions UI shows a generic label while building the container matrix.

Changes

Semantics are unchanged: same id: set-matrix, same matrix output wiring, and the same script (including the existing ::set-output usage).

• Add name: Build compatibility container matrix in compatiblity_test_on_pr.yml
• Add the same name in compatiblity_test_on_schedule.yml

Test plan

• Confirm matrix_preparation.outputs.matrix still comes from steps.set-matrix.outputs.matrix
• Confirm the matrix-building script is unchanged