const-ify `cover` #5634

Job	Run time
regression (macos-12)	29m 54s
benchcomp-tests	7m 30s
regression (ubuntu-20.04)	20m 37s
perf	27m 17s
write-json-symtab-regression	11m 5s
regression (ubuntu-22.04)	41m 12s
bookrunner	34m 32s
	2h 52m 7s

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