OPUS: Fix DTX detection #4560

Job	Run time
ci (ubuntu-20.04, gcc, g++, 20, Release)	8m 15s
ci (ubuntu-20.04, gcc, g++, 20, Debug)	7m 29s
ci (ubuntu-20.04, clang, clang++, 20, Release)	7m 59s
ci (ubuntu-20.04, clang, clang++, 20, Debug)	6m 54s
ci (ubuntu-22.04, gcc, g++, 20, Release)	7m 20s
ci (ubuntu-22.04, gcc, g++, 20, Debug)	6m 35s
ci (ubuntu-22.04, clang, clang++, 20, Release)	9m 9s
ci (ubuntu-22.04, clang, clang++, 20, Debug)	7m 41s
ci (macos-12, gcc, g++, 20, Release)	19m 37s
ci (macos-12, gcc, g++, 20, Debug)	12m 54s
ci (macos-14, clang, clang++, 20, Release)	4m 6s
ci (macos-14, clang, clang++, 20, Debug)	4m 6s
ci (windows-2022, cl, cl, 20, Release)	6m 57s
ci (windows-2022, cl, cl, 20, Debug)	6m 9s
	1h 55m 11s

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