Refactor the license header check script to not repeat the expected license header computation #106

Job	Run time
Test with Docker / Linux (5.8 - jammy)	26s
Test with Docker / Windows (5.9 - windows-2022)	6m 37s
Test without Docker / Windows (5.9 - windows-2022)	2m 52s
Test with Docker / Linux (5.9 - jammy)	26s
Test without Docker / Linux (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Test with Docker / Windows (6.0 - windows-2022)	6m 20s
Test without Docker / Windows (6.0 - windows-2022)	5m 47s
Test with Docker / Linux (5.10 - jammy)	30s
Test with Docker / Windows (nightly - windows-2019)	10m 1s
Test without Docker / Windows (nightly - windows-2022)	4m 43s
Test with Docker / Linux (6.0 - jammy)	37s
Test with Docker / Windows (nightly-6.1 - windows-2019)	11m 28s
Test without Docker / Windows (nightly-6.1 - windows-2022)	3m 28s
Test with Docker / Linux (nightly-main - jammy)	35s
Test with Docker / Linux (nightly-6.1 - jammy)	33s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	34s
Soundness / Format check	0s
Soundness / License headers check	3s
Soundness / Shell check	40s
Soundness / Python lint check	5s
Soundness / Unacceptable language check	4s
Soundness / YAML lint check	2s
	55m 54s

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