Unconditionally disable combining SARIF files for GHES 3.18 #4408

Job	Run time
Diagnostic export (macos-latest, linked)	1m 37s
Diagnostic export (windows-latest, nightly-latest)	4m 44s
Diagnostic export (ubuntu-latest, linked)	58s
Diagnostic export (macos-latest, nightly-latest)	1m 4s
Diagnostic export (windows-latest, linked)	3m 8s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 7s
	12m 38s

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