[CI] Enable Linux nightly main on PRs #100

Job	Run time
Unit tests / Construct unit test matrix	5s
Integration test / Windows (6.0)	1s
Integration test / Linux (5.9)	1m 47s
Integration test / Linux (5.10)	1m 31s
Integration test / Linux (6.0)	2m 2s
Integration test / Windows (nightly-6.0)	1s
Integration test / Linux (nightly-6.0)	2m 24s
Integration test / Windows (nightly-main)	1s
Soundness / API breakage check	54s
Integration test / Linux (nightly-main)	2m 35s
Soundness / Documentation check	1m 0s
Soundness / Unacceptable language check	4s
Soundness / Broken symlinks check	3s
Soundness / Format check	31s
Soundness / License headers check	6s
Soundness / Shell check	35s
Soundness / YAML lint check	4s
Soundness / Python lint check	3s
Unit tests / Unit tests / Linux (5.9)	47s
Unit tests / Unit tests / Linux (5.10)	43s
Unit tests / Unit tests / Linux (6.0)	49s
Unit tests / Unit tests / Linux (nightly-6.0)	59s
Unit tests / Unit tests / Linux (nightly-main)	58s
	18m 3s

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