Unconditionally disable combining SARIF files for GHES 3.18 #4410

Job	Run time
Diagnostic export (windows-latest, nightly-latest)	2m 31s
Diagnostic export (ubuntu-latest, linked)	57s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 3s
Diagnostic export (macos-latest, linked)	1m 0s
Diagnostic export (macos-latest, nightly-latest)	1m 4s
Diagnostic export (windows-latest, linked)	2m 38s
	9m 13s

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