Add attribute to Dummy: dummy_index #403

Job	Run time
build (Debug, yes, 3.11, yes, ubuntu-22.04, gcc)	3m 20s
build (Debug, yes, 3.12, yes, ubuntu-22.04, gcc)	3m 31s
build (Debug, yes, 3.9, yes, ubuntu-22.04, gcc)	3m 44s
build (Release, 3.9, yes, flint, yes, ubuntu-22.04, gcc)	5m 6s
build (Release, 3.13, yes, ubuntu-22.04, gcc)	4m 16s
build (Debug, 3.13, yes, yes, ubuntu-22.04, clang)	4m 2s
build (Release, 3.9, yes, ubuntu-22.04, gcc)	4m 29s
build (Debug, yes, 3.10, yes, ubuntu-22.04, gcc)	3m 42s
build (Debug, 3.13, yes, 5, macos-13, clang)	8m 8s
build (Release, 3.13, yes, ubuntu-22.04, clang)	4m 40s
build (Release, 3.13, yes, ubuntu-22.04, gcc)	4m 34s
build (Release, 3.13, yes, gmpxx, no, ubuntu-22.04, gcc)	4m 22s
build (Release, yes, 3.13, ubuntu-22.04, gcc)	4m 31s
build (Release, 3.13, macos-13, gcc)	5m 34s
build (Debug, 3.13, no, macos-13, gcc)	11m 24s
build (Release, 3.11, ubuntu-22.04, yes, yes, yes, yes, yes, yes, flint, yes, gcc)	5m 30s
build (Debug, 3.12, yes, 18, yes, yes, boostmp, -k "not integer_nthroot", ubuntu-24.04, gcc, llvm...	5m 26s
build (Release, 3.9, no, macos-13, clang)	5m 2s
	1h 31m 21s

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