rm: skip prompt when stdin is not interactive; Fix #7326 #6972

Job	Run time
Build the fuzzers	4m 13s
Run the fuzzers (fuzz_test, true)	2m 30s
Run the fuzzers (fuzz_printf, false)	2m 30s
Run the fuzzers (fuzz_expr, true)	2m 26s
Run the fuzzers (fuzz_date, false)	1m 29s
Run the fuzzers (fuzz_seq_parse_number, true)	2m 19s
Run the fuzzers (fuzz_parse_size, true)	1m 46s
Run the fuzzers (fuzz_echo, true)	2m 21s
Run the fuzzers (fuzz_wc, false)	1m 25s
Run the fuzzers (fuzz_cksum, false)	1m 31s
Run the fuzzers (fuzz_sort, false)	1m 27s
Run the fuzzers (fuzz_tr, false)	1m 38s
Run the fuzzers (fuzz_seq, false)	1m 44s
Run the fuzzers (fuzz_env, false)	1m 22s
Run the fuzzers (fuzz_parse_glob, true)	2m 20s
Run the fuzzers (fuzz_cut, false)	1m 22s
Run the fuzzers (fuzz_split, false)	1m 45s
Run the fuzzers (fuzz_parse_time, true)	2m 18s
	36m 26s

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