summarise_by_instance() only tries to truncate the common root path i… #177

Job	Run time
Check macos-13 (4)	7m 32s
Check macos-13 (release)	8m 38s
Check ubuntu-22.04 (4)	5m 0s
Check ubuntu-22.04 (release)	9m 19s
Check windows-latest (4)	5m 32s
Check windows-latest (release)	7m 8s
Check macos-14 (4)	3m 15s
Check macos-14 (release)	3m 37s
Check ubuntu-24.04 (devel)	5m 48s
	55m 49s

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