advice: refuse to output if stderr not TTY #1820

Job	Run time
config	5s
fuzz smoke test	33s
win+VS build	0s
static-analysis	4m 18s
linux32 (daald/ubuntu32:xenial)	5m 58s
linux-musl (alpine)	5m 55s
pedantic (fedora)	2m 7s
documentation	4m 13s
win build	6m 3s
linux-asan-ubsan (ubuntu-latest)	6m 10s
linux-gcc (ubuntu-20.04)	6m 9s
linux-gcc-default (ubuntu-latest)	6m 8s
linux-leaks (ubuntu-latest)	6m 8s
linux-reftable (ubuntu-latest)	6m 7s
linux-sha256 (ubuntu-latest)	6m 6s
linux-reftable-leaks (ubuntu-latest)	6m 8s
linux-TEST-vars (ubuntu-20.04)	6m 7s
osx-clang (macos-13)	6m 26s
osx-gcc (macos-13)	6m 9s
win+VS test	0s
osx-reftable (macos-13)	6m 16s
sparse	5m 42s
win test	0s
	1h 42m 48s

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