Don't save CI cache from secondary jobs, like mutants #1766

Job	Run time
quick-test	2m 23s
branch-mutants (cargo)	16s
cargo-mutants (0, cargo)	6m 14s
branch-mutants (nextest)	14s
pr-mutants	0s
cargo-mutants (1, cargo)	7m 33s
test (macOS-latest, stable)	5m 12s
cargo-mutants (2, cargo)	6m 47s
test (macOS-latest, nightly)	7m 52s
tests-from-tarball (ubuntu-latest)	2m 17s
cargo-mutants (3, cargo)	10m 23s
test (macOS-latest, 1.78)	5m 49s
cargo-mutants (4, cargo)	9m 2s
test (ubuntu-latest, stable)	4m 9s
cargo-mutants (5, cargo)	10m 46s
test (ubuntu-latest, nightly)	5m 7s
cargo-mutants (6, cargo)	8m 30s
test (ubuntu-latest, 1.78)	4m 53s
cargo-mutants (7, cargo)	11m 9s
test (windows-latest, stable)	7m 22s
test (windows-latest, nightly)	10m 4s
cargo-mutants (8, cargo)	9m 56s
cargo-mutants (9, cargo)	6m 47s
test (windows-latest, 1.78)	8m 22s
	2h 31m 7s

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