Don't log from inside the logger
This deadlocks ProgressBar, e.g.
# nix run --impure --no-substitute --store '/tmp/nix2?store=/foo' --expr 'derivation { builder = /nix/store/zi90rxslsm4mlr46l2xws1rm94g7pk8p-busybox-1.31.1-x86_64-unknown-linux-musl/bin/busybox; }'
leads to
Thread 1 (Thread 0x7ffff6126e80 (LWP 12250)):
#0 0x00007ffff7215d62 in __lll_lock_wait () from /nix/store/9df65igwjmf2wbw0gbrrgair6piqjgmi-glibc-2.31/lib/libpthread.so.0
#1 0x00007ffff720e721 in pthread_mutex_lock () from /nix/store/9df65igwjmf2wbw0gbrrgair6piqjgmi-glibc-2.31/lib/libpthread.so.0
#2 0x00007ffff7ad17fa in __gthread_mutex_lock (__mutex=0x6c5448) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/x86_64-unknown-linux-gnu/bits/gthr-default.h:749
#3 std::mutex::lock (this=0x6c5448) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/bits/std_mutex.h:100
#4 std::unique_lock<std::mutex>::lock (this=0x7fffffff09a8, this=0x7fffffff09a8) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/bits/unique_lock.h:141
#5 std::unique_lock<std::mutex>::unique_lock (__m=..., this=0x7fffffff09a8) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/bits/unique_lock.h:71
#6 nix::Sync<nix::ProgressBar::State, std::mutex>::Lock::Lock (s=0x6c5448, this=0x7fffffff09a0) at src/libutil/sync.hh:45
#7 nix::Sync<nix::ProgressBar::State, std::mutex>::lock (this=0x6c5448) at src/libutil/sync.hh:85
#8 nix::ProgressBar::logEI (this=0x6c5440, ei=...) at src/libmain/progress-bar.cc:131
#9 0x00007ffff7608cfd in nix::Logger::logEI (ei=..., lvl=nix::lvlError, this=0x6c5440) at src/libutil/logging.hh:88
#10 nix::getCodeLines (errPos=...) at src/libutil/error.cc:66
#11 0x00007ffff76073f2 in nix::showErrorInfo (out=..., einfo=..., showTrace=<optimized out>) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/optional:897
#12 0x00007ffff7ad19e7 in nix::ProgressBar::logEI (this=0x6c5440, ei=...) at src/libmain/progress-bar.cc:134
#13 0x00007ffff7ab9d10 in nix::Logger::logEI (ei=..., lvl=nix::lvlError, this=0x6c5440) at src/libutil/logging.hh:88
#14 nix::handleExceptions(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::function<void ()>) (programName="/home/eelco/Dev/nix/outputs/out/bin/nix", fun=...) at src/libmain/shared.cc:328
#15 0x000000000046226b in main (argc=<optimized out>, argv=<optimized out>) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/ext/new_allocator.h:80
724b7f4fb6