Allow configuring CODEQL_THREADS with an env var #4154

Job	Run time
Diagnostic export (macos-latest, linked)	20s
Diagnostic export (ubuntu-latest, linked)	13s
Diagnostic export (ubuntu-latest, nightly-latest)	12s
Diagnostic export (windows-latest, linked)	38s
Diagnostic export (macos-latest, nightly-latest)	16s
Diagnostic export (windows-latest, nightly-latest)	44s
	2m 23s

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