Update generate matrix stript to use nightly_next #1370

Job	Run time
Construct integration test matrix	4s
Vsock tests	2m 24s
Benchmarks / Construct Benchmarks matrix	3s
Unit tests / Construct unit test matrix	5s
Cxx interop / Construct Cxx interop matrix	3s
Soundness / API breakage check	1m 33s
Soundness / Broken symlinks check	5s
Soundness / Documentation check	1m 31s
Soundness / License headers check	7s
Soundness / Unacceptable language check	4s
Soundness / Format check	44s
Soundness / Python lint check	7s
Soundness / Shell check	35s
Soundness / YAML lint check	5s
Cxx interop / Cxx interop / Linux (5.9)	1m 13s
Cxx interop / Cxx interop / Linux (5.10)	1m 11s
Cxx interop / Cxx interop / Linux (6.0)	1m 24s
Cxx interop / Cxx interop / Linux (nightly-6.1)	1m 34s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 30s
Benchmarks / Benchmarks / Linux (5.9)	1m 28s
Benchmarks / Benchmarks / Linux (5.10)	1m 28s
Benchmarks / Benchmarks / Linux (6.0)	1m 29s
Benchmarks / Benchmarks / Linux (nightly-next)	1m 28s
Benchmarks / Benchmarks / Linux (nightly-main)	1m 28s
Unit tests / Unit tests / Linux (5.9)	1m 28s
Unit tests / Unit tests / Linux (5.10)	1m 28s
Unit tests / Unit tests / Linux (6.0)	1m 27s
Unit tests / Unit tests / Linux (nightly-next)	1m 25s
Unit tests / Unit tests / Linux (nightly-main)	1m 26s
	28m 57s

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