Suppress ExecutionContext flow in BeginErrorReadLine #2650

Job	Run time
build (macos-latest, Debug)	8m 45s
build (ubuntu-latest, Debug)	10m 50s
build (windows-latest, Debug)	17m 21s
build (ubuntu-latest, Release)	10m 11s
build (macos-latest, Release)	9m 43s
build (windows-latest, Release)	15m 12s
publish-coverage / publish-coverage	28s
	1h 12m 30s

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