Add attribute to Dummy: dummy_index #398

Job	Run time
build (Debug, yes, 3.11, yes, ubuntu-22.04, gcc)	3m 21s
build (Release, 3.13, yes, gmpxx, no, ubuntu-22.04, gcc)	4m 15s
build (Debug, yes, 3.12, yes, ubuntu-22.04, gcc)	3m 34s
build (Debug, yes, 3.10, yes, ubuntu-22.04, gcc)	3m 29s
build (Release, yes, 3.13, ubuntu-22.04, gcc)	4m 9s
build (Debug, yes, 3.9, yes, ubuntu-22.04, gcc)	3m 22s
build (Release, 3.9, yes, flint, yes, ubuntu-22.04, gcc)	4m 33s
build (Debug, 3.13, yes, yes, ubuntu-22.04, clang)	3m 49s
build (Release, 3.13, yes, ubuntu-22.04, gcc)	4m 15s
build (Release, 3.9, yes, ubuntu-22.04, gcc)	4m 29s
build (Release, 3.13, yes, ubuntu-22.04, gcc)	4m 32s
build (Debug, 3.13, yes, 5, macos-13, clang)	5m 47s
build (Debug, 3.12, yes, 18, yes, yes, boostmp, -k "not integer_nthroot", ubuntu-24.04, gcc, llvm...	5m 37s
build (Debug, 3.13, no, macos-13, gcc)	5m 53s
build (Release, 3.11, ubuntu-22.04, yes, yes, yes, yes, yes, yes, flint, yes, gcc)	4m 46s
build (Release, 3.13, macos-13, gcc)	7m 30s
build (Release, 3.9, no, macos-13, clang)	5m 58s
build (Release, 3.13, yes, ubuntu-22.04, clang)	4m 22s
	1h 23m 41s

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