env: use Command::exec() instead of libc::execvp() #14530

Job	Run time
Build the fuzzers	1m 13s
Build and test uufuzz examples	31s
Fuzz (fuzz_date, false)	1m 8s
Fuzz (fuzz_test, true)	2m 10s
Fuzz (fuzz_printf, true)	2m 8s
Fuzz (fuzz_seq, false)	1m 8s
Fuzz (fuzz_wc, false)	1m 5s
Fuzz (fuzz_expr, true)	2m 8s
Fuzz (fuzz_echo, true)	2m 17s
Fuzz (fuzz_sort, false)	1m 14s
Fuzz (fuzz_split, false)	1m 9s
Fuzz (fuzz_cut, false)	1m 7s
Fuzz (fuzz_parse_glob, true)	2m 4s
Fuzz (fuzz_seq_parse_number, true)	2m 3s
Fuzz (fuzz_env, false)	1m 7s
Fuzz (fuzz_parse_size, true)	59s
Fuzz (fuzz_parse_time, true)	2m 9s
Fuzz (fuzz_non_utf8_paths, true)	2m 7s
Fuzz (fuzz_cksum, false)	1m 4s
Fuzz (fuzz_tr, false)	1m 21s
Fuzzing Summary	8s
	30m 20s

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