Simplify code to list files and mutants #1638

Job	Run time
minimal-versions (macOS-latest)	3m 22s
test (macOS-latest, stable)	2m 20s
minimal-versions (ubuntu-latest)	1m 52s
minimal-versions (windows-latest)	3m 35s
install (macOS-latest, stable)	1m 59s
test (macOS-latest, nightly)	2m 19s
install (macOS-latest, nightly)	50s
test (macOS-latest, 1.74)	2m 37s
install (macOS-latest, 1.74)	1m 59s
maximal-versions (macOS-latest)	2m 46s
install (ubuntu-latest, stable)	1m 21s
test (ubuntu-latest, stable)	2m 7s
maximal-versions (ubuntu-latest)	1m 50s
install (ubuntu-latest, nightly)	50s
test (ubuntu-latest, nightly)	1m 41s
install (ubuntu-latest, 1.74)	1m 22s
maximal-versions (windows-latest)	3m 53s
test (ubuntu-latest, 1.74)	2m 27s
install (windows-latest, stable)	3m 43s
install (windows-latest, nightly)	2m 7s
install (windows-latest, 1.74)	2m 44s
release-binary	25s
test (windows-latest, stable)	4m 30s
test (windows-latest, nightly)	3m 51s
tests-from-tarball (ubuntu-latest)	2m 13s
test (windows-latest, 1.74)	3m 40s
Spell check with Typos	3s
pr-mutants (cargo)	3m 27s
pr-mutants (nextest)	3m 12s
cargo-mutants (0, cargo)	7m 5s
cargo-mutants (1, cargo)	6m 13s
cargo-mutants (2, cargo)	13m 2s
cargo-mutants (3, cargo)	10m 19s
cargo-mutants (4, cargo)	11m 19s
cargo-mutants (5, cargo)	10m 37s
cargo-mutants (6, cargo)	10m 29s
cargo-mutants (7, cargo)	12m 9s
cargo-mutants (8, cargo)	9m 47s
cargo-mutants (9, cargo)	10m 38s
	2h 50m 43s

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