Merge pull request #276 from patstew/ccache-fix #335

Job	Run time
build	1m 9s
build-spm	1m 3s
docs	3m 46s
check (Debug, gnu-6, 14)	8m 6s
check (Debug, gnu-7, 14)	8m 25s
check (Debug, llvm-5, 14)	6m 32s
check (Debug, llvm-6, 14)	6m 33s
check (Release, gnu-6, 14)	10m 22s
check (Release, gnu-7, 14)	11m 13s
check (Release, llvm-5, 14)	11m 32s
check (Release, llvm-6, 14)	12m 28s
check (Debug, gnu-9, 14, coverage)	10m 49s
check (Debug, gnu-8, 17)	8m 25s
check (Debug, llvm-9, 17, fuzzers)	8m 32s
check (Debug, gnu-11, 20)	9m 19s
check (Debug, llvm-13, 20)	6m 57s
check (Debug, llvm-8, 14, sanitize)	10m 49s
check (Release, gnu-9, 14, benchmark)	29m 22s
check (Release, llvm-10, 14, benchmark)	30m 13s
	3h 15m 35s

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