Add attribute to Dummy: dummy_index #401

Job	Run time
build (Release, 3.13, yes, ubuntu-22.04, gcc)	3m 16s
build (Release, 3.13, yes, gmpxx, no, ubuntu-22.04, gcc)	3m 21s
build (Debug, yes, 3.11, yes, ubuntu-22.04, gcc)	3m 8s
build (Debug, yes, 3.9, yes, ubuntu-22.04, gcc)	4m 29s
build (Debug, yes, 3.10, yes, ubuntu-22.04, gcc)	3m 5s
build (Debug, yes, 3.12, yes, ubuntu-22.04, gcc)	3m 38s
build (Release, yes, 3.13, ubuntu-22.04, gcc)	4m 40s
build (Release, 3.9, yes, ubuntu-22.04, gcc)	3m 24s
build (Debug, 3.13, yes, yes, ubuntu-22.04, clang)	3m 47s
build (Release, 3.13, yes, ubuntu-22.04, clang)	4m 13s
build (Debug, 3.13, no, macos-13, gcc)	5m 59s
build (Release, 3.9, no, macos-13, clang)	4m 35s
build (Release, 3.11, ubuntu-22.04, yes, yes, yes, yes, yes, yes, flint, yes, gcc)	5m 10s
build (Debug, 3.12, yes, 18, yes, yes, boostmp, -k "not integer_nthroot", ubuntu-24.04, gcc, llvm...	4m 58s
build (Release, 3.13, macos-13, gcc)	5m 34s
build (Release, 3.13, yes, ubuntu-22.04, gcc)	4m 3s
build (Debug, 3.13, yes, 5, macos-13, clang)	7m 46s
build (Release, 3.9, yes, flint, yes, ubuntu-22.04, gcc)	3m 17s
	1h 18m 23s

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