Allow configuring CODEQL_THREADS with an env var #4809

Job	Run time
Ruby analysis (ubuntu-latest, default)	43s
Ruby analysis (macos-latest, linked)	1m 19s
Ruby analysis (ubuntu-latest, linked)	45s
Ruby analysis (macos-latest, default)	52s
Ruby analysis (macos-latest, nightly-latest)	49s
Ruby analysis (ubuntu-latest, nightly-latest)	44s
	5m 12s

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