Cargo.toml: enable uucore's `mode` feature #3182

Job	Run time
Build the fuzzers	3m 45s
Run the fuzzers (fuzz_test, true)	4m 14s
Run the fuzzers (fuzz_date, false)	4m 6s
Run the fuzzers (fuzz_expr, true)	4m 8s
Run the fuzzers (fuzz_printf, false)	3m 17s
Run the fuzzers (fuzz_echo, true)	4m 14s
Run the fuzzers (fuzz_seq, false)	3m 11s
Run the fuzzers (fuzz_sort, false)	3m 50s
Run the fuzzers (fuzz_wc, false)	3m 19s
Run the fuzzers (fuzz_cut, false)	3m 19s
Run the fuzzers (fuzz_split, false)	3m 37s
Run the fuzzers (fuzz_tr, false)	3m 22s
Run the fuzzers (fuzz_env, false)	3m 9s
Run the fuzzers (fuzz_parse_glob, true)	4m 5s
Run the fuzzers (fuzz_parse_size, true)	4m 5s
Run the fuzzers (fuzz_parse_time, true)	4m 9s
Run the fuzzers (fuzz_seq_parse_number, true)	4m 6s
	1h 3m 56s

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