Allow specifying a tag that should be used as a baseline for API breakage checks #111

Job	Run time
Test without Docker / Linux (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Test with Docker / Linux (5.8 - jammy)	26s
Test without Docker / Windows (5.9 - windows-2022)	5m 43s
Test with Docker / Linux (5.9 - jammy)	28s
Test with Docker / Windows (5.9 - windows-2022)	6m 33s
Test without Docker / Windows (6.0 - windows-2022)	5m 6s
Test with Docker / Linux (5.10 - jammy)	28s
Test with Docker / Windows (6.0 - windows-2022)	11m 23s
Test without Docker / Windows (nightly - windows-2022)	5m 15s
Test with Docker / Linux (6.0 - jammy)	32s
Test with Docker / Windows (nightly - windows-2019)	8m 46s
Test without Docker / Windows (nightly-6.1 - windows-2022)	5m 5s
Test with Docker / Linux (nightly-main - jammy)	40s
Test with Docker / Windows (nightly-6.1 - windows-2019)	9m 38s
Test with Docker / Linux (nightly-6.1 - jammy)	39s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	36s
Soundness / Format check	0s
Soundness / License headers check	3s
Soundness / Shell check	30s
Soundness / Unacceptable language check	4s
Soundness / Python lint check	4s
Soundness / YAML lint check	5s
	1h 2m 7s

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