check-matrix-job.sh should tolerate unset 6.1 vars #1256

Job	Run time
Construct integration test matrix	3s
Vsock tests	2m 6s
Benchmarks / Construct Benchmarks matrix	3s
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Construct unit test matrix	2s
Soundness / API breakage check	2m 4s
Soundness / Broken symlinks check	2s
Soundness / Documentation check	2m 12s
Soundness / Format check	50s
Soundness / License headers check	7s
Soundness / Python lint check	9s
Soundness / Shell check	38s
Soundness / Unacceptable language check	3s
Soundness / YAML lint check	4s
Cxx interop / Cxx interop / Linux (5.9)	1m 8s
Integration tests / Linux (5.9)	7m 38s
Benchmarks / Benchmarks / Linux (5.9)	5m 20s
Cxx interop / Cxx interop / Linux (5.10)	1m 8s
Integration tests / Linux (5.10)	7m 37s
Benchmarks / Benchmarks / Linux (5.10)	5m 14s
Cxx interop / Cxx interop / Linux (6.0)	1m 17s
Integration tests / Linux (6.0)	7m 39s
Benchmarks / Benchmarks / Linux (6.0)	5m 41s
Cxx interop / Cxx interop / Linux (nightly-6.1)	1m 41s
Benchmarks / Benchmarks / Linux (nightly-6.1)	4m 27s
Integration tests / Linux (nightly-6.1)	7m 38s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 35s
Integration tests / Linux (nightly-main)	7m 39s
Benchmarks / Benchmarks / Linux (nightly-main)	4m 28s
Unit tests / Unit tests / Linux (5.9)	3m 57s
Unit tests / Unit tests / Linux (5.10)	3m 55s
Unit tests / Unit tests / Linux (6.0)	4m 15s
Unit tests / Unit tests / Linux (nightly-6.1)	4m 35s
Unit tests / Unit tests / Linux (nightly-main)	4m 47s
	1h 40m 5s

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