[CI] Enable Linux nightly main on PRs #99

Job	Run time
Unit tests / Construct unit test matrix	5s
Integration test / Windows (6.0)	2s
Integration test / Windows (nightly-6.0)	2s
Integration test / Linux (5.9)	1m 47s
Integration test / Windows (nightly-main)	1s
Soundness / API breakage check	57s
Soundness / Broken symlinks check	12s
Integration test / Linux (5.10)	1m 33s
Integration test / Linux (6.0)	2m 0s
Soundness / Documentation check	1m 7s
Integration test / Linux (nightly-6.0)	2m 14s
Soundness / Format check	32s
Soundness / License headers check	10s
Integration test / Linux (nightly-main)	2m 25s
Soundness / Unacceptable language check	5s
Soundness / Python lint check	3s
Soundness / YAML lint check	5s
Soundness / Shell check	33s
Unit tests / Unit tests / Linux (5.9)	41s
Unit tests / Unit tests / Linux (5.10)	45s
Unit tests / Unit tests / Linux (6.0)	49s
Unit tests / Unit tests / Linux (nightly-6.0)	1m 5s
Unit tests / Unit tests / Linux (nightly-main)	58s
	18m 11s

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