CI use 6.1 nightlies #22

Job	Run time
Cxx interop / Construct Cxx interop matrix	2s
Unit tests / Unit tests (5.9)	1m 18s
Unit tests / Unit tests (5.10)	1m 17s
Unit tests / Unit tests (6.0)	1m 32s
Unit tests / Unit tests (nightly-6.1)	1m 54s
Unit tests / Unit tests (nightly-main)	1m 47s
Soundness / Documentation check	1m 36s
Soundness / Broken symlinks check	4s
Soundness / API breakage check	1m 42s
Soundness / Format check	31s
Soundness / License headers check	3s
Soundness / Python lint check	5s
Soundness / Shell check	34s
Soundness / Unacceptable language check	5s
Soundness / YAML lint check	5s
Cxx interop / Cxx interop / Linux (5.9)	1m 21s
Cxx interop / Cxx interop / Linux (5.10)	1m 12s
Cxx interop / Cxx interop / Linux (6.0)	1m 19s
Cxx interop / Cxx interop / Linux (nightly-6.1)	1m 46s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 30s
	19m 43s

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