Allow configuring CODEQL_THREADS with an env var #4139

Job	Run time
Diagnostic export (ubuntu-latest, linked)	1m 4s
Diagnostic export (windows-latest, linked)	6m 10s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 5s
Diagnostic export (macos-latest, nightly-latest)	1m 39s
Diagnostic export (macos-latest, linked)	1m 19s
Diagnostic export (windows-latest, nightly-latest)	3m 33s
	14m 50s

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