probe for root dir instead of guessing #4294

Job	Run time
shellcheck	11s
cargo-deny	17s
clippy (macos-latest)	23s
clippy (ubuntu-latest)	21s
check	15s
clippy (windows-latest)	51s
fmt	6s
generate-matrix	34s
test (macos-latest)	3m 36s
test (ubuntu-latest)	3m 11s
test (windows-latest)	5m 4s
foreign	4m 53s
bisect	6m 55s
docker-in-docker	5m 58s
target (${{ matrix.pretty }},${{ matrix.os }})	0s
remote	4m 31s
podman	4m 45s
publish	0s
conclusion	1s
Coverage	2m 25s
	44m 17s

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